NON-OSMOTIC REGULATION OF RENAL WATER EXCRETION* ROBERT W. SCHRIER, M.D., AND (by invitation) TOMAS BERL, M.D., ROBERT J. ANDERSON, M.D. AND KEITH M. McDONALD M.D. DENVER, COLORADO
In normal circumstances, the osmoreceptor-antidiuretic hormone (ADH) system exquisitely regulates renal water excretion in response to alterations in water intake, so that the plasma sodium concentration remains within the normal range.I There are, however, a number of pathophysiological circumstances in which a diminution in plasma sodium concentration occurs. In these hyponatremic settings either dysfunction of the osmoreceptor-ADH system is present or non-osmotic stimuli predominate over osmotic stimuli.2 It has been postulated that the autonomic nervous system may be involved in this non-osmotic regulation of ADH release.3' In this regard, beta adrenergic stimulation with intravenous isoproterenol has been demonstrated to cause an antidiuresis in rat,4 dog5 and man,6 while alpha adrenergic stimulation causes a diuresis in the same species.4'6'7 We have performed a series of experiments in our laboratory to further define the role of the autonomic nervous system in the non-osmotic regulation of renal water excretion. The beta agonist, isoproterenol, was found to cause an antidiuresis in anesthetized normal rats but not in Brattleboro rats suffering from hereditary pituitary diabetes insipidus. Further support for the role of endogenous vasopressin in mediating the effect of beta adrenergic stimulation is the finding that isoproterenol increases the renal medullary concentration of adenosine 3'5'monophosphate (cyclic AMP) in intact but not Brattleboro rats (Fig. 1). Also, alpha adrenergic stimulation with norepinephrine suppresses urinary osmolality and medullary cyclic AMP in intact rats but not in Brattleboro rats receiving an exogenous infusion of vasopressin. Thus, since cyclic AMP is the known intracellular "secondary messenger" of the hydro-osmotic action of vasopressin, alterations in the release of endogenous vasopressin seems to be involved in modulating the effects of alpha- and beta-adrenergic stimulation on renal water excretion. Experimental studies also have been undertaken to delineate the pathway whereby alpha- and beta-adrenergic stimulation alter the en* From the Division of Renal Diseases, Department of Medicine, University of Colorado School of Medicine, Denver, Colorado. These studies were supported by grants from the National Institutes of Health (HL 15467 and HL 15629). 161
162
SCHRIER ET AL.
Control
.W
Isoproterenol Isoproterenol in Diabetes Insipidus intactin Rats Rats
.
U.'0 4) CU
IE t-
=-,r
FIG. 1. Effect of intravenous isoproterenol on renal medullary cyclic AMP in intact rats and rats suffering from diabetes insipidus.
dogenous release of vasopressin. Direct infusion of these catecholamines into the carotid circulation failed to alter renal water excretion, thus suggesting the involvement of an extracerebral reflex mechanism.8' Since alterations in perfusion pressure at the level of the carotid baroreceptors influence renal water excretion, studies were performed in animals with baroreceptor denervation. Bilateral cervical vagotomy failed to abolish the antidiuresis of isoproterenol, thus excluding a role of the low pressure atrial baroreceptors in modulating this effect on renal water excretion (Fig. 2). The additional denervation of the high pressure carotid baroreceptors, however, did abolish the antidiuretic effect of isoproterenol (Fig. 2).8 We also have found that the diuretic effect of intravenous norepinephrine is abolished by denervation of arterial baroreceptors.9 The results of these above studies suggested that baroreceptor tone may modulate the non-osmotic release of ADH by altering parasympathetic afferent tone to the hypothalamus. Parasympathetic efferent pathways do not seem to be involved since their blockade with atropine does not alter renal water excretion.10 If parasympathetic afferent neurons constitute an important pathway for the regulation of nonosmotic ADH release, then circulatory alterations other than those resulting from systemic alpha- and beta-adrenergic stimulation might produce similar alterations in renal water excretion. Studies therefore were performed in which cardiac output was lowered acutely by constricting the thoracic inferior vena cava.11 In these studies cardiac output was diminished approximately 30% as renal perfusion pressure was kept constant by adjustment of a suprarenal aortic clamp. This
163
REGULATION OF RENAL WATER EXCRETION Sham-Operated
Pt.Contriol
Intravenoos lopreretol
Denervated Baroreceptors
Vagotornzed Post-Control
Pr-otw Pta-Control
Intravetwos stntrowr.-
Post-Control
Pte-Control
Intratrnol
Po Control
SW0 400 wooena
UnaY Ogmoaloty
(mOsr/kg H20
300
200 100
*60
Free-Water .40 Clearance (ml/min)
>1
.20
0-2£0
FIG. 2. Antidiuretic effect of intravenous isoproterenol in sham operated (left panel) and vagotomized (middle panel) dogs. Isoproterenol did not alter urinary osmolality or freewater clearance in animals with denervated arterial baroreceptors (right panel).
circulatory alteration was associated with a mean diminution in arterial pressure of approximately 25 mm Hg at the level of the baroreceptors and a consistent antidiuresis (Fig. 3). This antidiuretic effect was virtually abolished by prior removal of the pituitary source of vasopressin. In these studies an acute hypophysectomy via the buccal apptoach was performed prior to the experiments and the animals were replaced with glucocorticoid hormone. In these animals thoracic inferior vena caval constriction altered cardiac output and arterial pressure to a degree similar to that observed in the intact, control dogs. It was then examined whether this non-osmotic release of ADH was mediated by intact baroreceptors. The studies therefore were repeated in animals with an intact pituitary source of ADH release but with denervated arterial baroreceptors, i.e., bilateral section of both vagal and glossopharyngeal nerves. Acute lowering Of cardiac output and arterial pressure of thoracic caval constriction did not produce an antidiuresis in these experiments (Fig. 3). The results of these above series of experiments thus indicated that alteration in baroreceptor tone in the arterial vascular tree constitutes an important non-osmotic pathway for ADH release. Additional studies have demonstrated that even the antidiuretic effect of intravenous nicotine is dependent on intact parasympathetic pathways rather than exerting a direct effect on the hypothalamic release of ADH as previously suggested. 12
164
SCHRIER ET AL. Intact Neuro-Hypophyseal Tract
Hypophysectomized
Thso,ac .c Vena Ca ConstS, c ,on
Tho, &c c
L
I
Pre Control
1001
Pre Control
Urinary
,
Denervated Baroreceptors I
Posr Control
Pm Contl
I hor&C.c
IV.n. Ca..
I
Post Control
Consor.cton
/
600 Osmolal,ty 600 (mOsm kg H2O)
2001
Post Conrol
Infe,o, Vens C"av
lo Cos
iill
//_
/
km
i "N
Free Water 2 c Clearance (ml 'min) 2(
FIG. 3. Antidiuretic ef'fect of' acute thoracic inferior vena cava constriction on renal water excretion in intact dogs (left panel). Removal of the source of ADH release in the posterior pituitary (middle panel) or baroreceptor denervation (right panel) virtually abolishes this antidiuresis of thoracic caval constriction.
In view of' the previous studies suggesting the presence of' low pressure baroreceptors in the atrium, 13, 14 further studies were performed to examine whether increases in left atrial pressure during rapid atrial pacing were involved in the non-osmotic regulation of ADH release.15 Atrial pacing to increase heart rate by approximately 100 beats/min was f'ound to cause a consistent increase in renal water excretion (Fig. 4). This degree of atrial pacing raised mean left atrial pressure approximately 10 mm Hg in both intact and acutely hypophysectomized, steroid replaced animals. When the hypophysectomized animals received a constant infusion of' vasopressin throughout the experiments, atrial pacing did not increase renal water excretion. Moreover, bilateral cervical vagotomy also abolished this diuretic response to atrial pacing. Since the carotid sinus baroreceptors were intact and atrial pacing caused a modest (5-10 mm Hg) fall in arterial pressure (an effect which if anything would stimulate ADH release and cause an antidiuresis), the water diuresis observed with atrial pacing seemed to be mediated by alterations in parasympathetic afferent pathways from the low pressure system with subsequent suppression of' endogenous vasopressin release. In this regard, an increase in left atrial pressure by inflation of' an intra-atrial baloon also is associated with increased vagal tone and a water diuresis that is abolished by bilateral cervical vagotomy.16 Taken together, the results of the above studies indicate that the nonosmotic release of ADH is modulated by alterations in baroreceptor in both low (atrial) and high (arterial) pressure systems. In some
165
REGULATION OF RENAL WATER EXCRETION Intact Animals Pre Conlrol
1500
Atrial
Pacorg
Post Control
Hypophysectomized Animals Pre Control
Atrial
Pacing
Post Control
Animals with Cervical Vagotomy Pie Conrol
IPacing
Post Control
Urinary 1000 Osmolality
(m0sm/kg H20) 500
+1 0-
+0 5
Free-Water Clearance
OO
fcc/min) -0 5
-1 0
A
0__
..a
:.11.
FIG. 4. Diuretic effect of atrial pacing in dogs (left panel) and the effect of hypophysectomy (middle panel) and bilateral cervical vagotomy (right panel) to abolish this diuresis.
circumstances both low and high pressure baroreceptors may be activated, such as during an acute hemorrhage or an acute reduction in cardiac output. On the other hand, the high and low pressure baroreceptors may receive opposing stimuli, such as during atrial pacing. In this setting the left atrial receptors may prove to be more sensitive. However, with more rapid atrial pacing and larger decrements in arterial pressure, the high pressure baroreceptors may be more potent and predominate, thereby causing an antidiuresis. Circulatory disturbances which alter baroreceptor tone in the low or high pressure vascular system or both, thus provide important nonosmotic stimuli for ADH release. From a phylogenetic viewpoint, this non-osmotic release of ADH may constitute an integral part of the alarm reaction and in this setting the vascular effect of vasopressin may have been as important as its water conserving action, particularly in sea-f'aring species. 17 While the release of ADH seems to constitute an important nonosmotic factor inf'luencing renal water excretion, there are also intrarenal f'actors which are important determinants of' renal water excretion, including renal arterial pressure, glomerular filtration rate, renal blood
166
SCHRIER ET AL.
flow and solute excretion. In addition to these factors we have examined in vivo the role of renal prostaglandins in modulating the hydrosmotic effect of vasopressin. Previous in vitro experiments in anuran membranes had suggested that prostaglandin of the E series impair the effect of vasopressin to both increase cyclic AMP and enhance osmotic water movement. 18-21 In the present experiments two sequential 100 mU boluses of vasopressin were injected into anesthetized dogs undergoing a water diuresis. Between the two boluses of vasopressin either a blank soluition or an inhibitor of prostaglandin synthesis (indomethacin or meclofenamate) was administered.22 As shown in Figure 5, the repeated doses of vasopressin, with the blank solution intervening, produced a comparable antidiuresis; but the administration of either m-eclofenamate or indomethacin markedly enhanced the antidiuretic effect of vasopressin. The results were compatible with the interpretation that renal medullary prostaglandins do inhibit the hydrosmotic action of vasopressin. This effect of prostaglandins could be mediated by regulating medullary blood flow, and thus influencing the rate of medullary washout of the hypertonic interstitium, or by impairing the vasopressinmediated generation of cyclic AMP. In regards to the latter possibility, it is important to emphasize that medullary collecting duct cells have been shown by histochemical techniques to contain prostaglandin.23 Several other factors also favor the mechanism involving cyclic AMP. One of us 41
Urinary
Osmolaity
(rnOsm/kg H20)
* Blank Solution *.. Indomnethacen 0---0 Meclofenamate
FIG. 5. Effect of inhibition of prostaglandin synthesis with indomethacin or meclofenamate to enhance the antidiuretic effect of vasopressin.
o
REGULATION OF RENAL WATER EXCRETION
I
1.
21,
u CL
14 p
In normal circumstances, the osmoreceptor-antidiuretic hormone (ADH) system exquisitely regulates renal water excretion in response to alterations in water intake, so that the plasma sodium concentration remains within the normal range.I There are, however, a number of pathophysiological circumstances in which a diminution in plasma sodium concentration occurs. In these hyponatremic settings either dysfunction of the osmoreceptor-ADH system is present or non-osmotic stimuli predominate over osmotic stimuli.2 It has been postulated that the autonomic nervous system may be involved in this non-osmotic regulation of ADH release.3' In this regard, beta adrenergic stimulation with intravenous isoproterenol has been demonstrated to cause an antidiuresis in rat,4 dog5 and man,6 while alpha adrenergic stimulation causes a diuresis in the same species.4'6'7 We have performed a series of experiments in our laboratory to further define the role of the autonomic nervous system in the non-osmotic regulation of renal water excretion. The beta agonist, isoproterenol, was found to cause an antidiuresis in anesthetized normal rats but not in Brattleboro rats suffering from hereditary pituitary diabetes insipidus. Further support for the role of endogenous vasopressin in mediating the effect of beta adrenergic stimulation is the finding that isoproterenol increases the renal medullary concentration of adenosine 3'5'monophosphate (cyclic AMP) in intact but not Brattleboro rats (Fig. 1). Also, alpha adrenergic stimulation with norepinephrine suppresses urinary osmolality and medullary cyclic AMP in intact rats but not in Brattleboro rats receiving an exogenous infusion of vasopressin. Thus, since cyclic AMP is the known intracellular "secondary messenger" of the hydro-osmotic action of vasopressin, alterations in the release of endogenous vasopressin seems to be involved in modulating the effects of alpha- and beta-adrenergic stimulation on renal water excretion. Experimental studies also have been undertaken to delineate the pathway whereby alpha- and beta-adrenergic stimulation alter the en* From the Division of Renal Diseases, Department of Medicine, University of Colorado School of Medicine, Denver, Colorado. These studies were supported by grants from the National Institutes of Health (HL 15467 and HL 15629). 161
162
SCHRIER ET AL.
Control
.W
Isoproterenol Isoproterenol in Diabetes Insipidus intactin Rats Rats
.
U.'0 4) CU
IE t-
=-,r
FIG. 1. Effect of intravenous isoproterenol on renal medullary cyclic AMP in intact rats and rats suffering from diabetes insipidus.
dogenous release of vasopressin. Direct infusion of these catecholamines into the carotid circulation failed to alter renal water excretion, thus suggesting the involvement of an extracerebral reflex mechanism.8' Since alterations in perfusion pressure at the level of the carotid baroreceptors influence renal water excretion, studies were performed in animals with baroreceptor denervation. Bilateral cervical vagotomy failed to abolish the antidiuresis of isoproterenol, thus excluding a role of the low pressure atrial baroreceptors in modulating this effect on renal water excretion (Fig. 2). The additional denervation of the high pressure carotid baroreceptors, however, did abolish the antidiuretic effect of isoproterenol (Fig. 2).8 We also have found that the diuretic effect of intravenous norepinephrine is abolished by denervation of arterial baroreceptors.9 The results of these above studies suggested that baroreceptor tone may modulate the non-osmotic release of ADH by altering parasympathetic afferent tone to the hypothalamus. Parasympathetic efferent pathways do not seem to be involved since their blockade with atropine does not alter renal water excretion.10 If parasympathetic afferent neurons constitute an important pathway for the regulation of nonosmotic ADH release, then circulatory alterations other than those resulting from systemic alpha- and beta-adrenergic stimulation might produce similar alterations in renal water excretion. Studies therefore were performed in which cardiac output was lowered acutely by constricting the thoracic inferior vena cava.11 In these studies cardiac output was diminished approximately 30% as renal perfusion pressure was kept constant by adjustment of a suprarenal aortic clamp. This
163
REGULATION OF RENAL WATER EXCRETION Sham-Operated
Pt.Contriol
Intravenoos lopreretol
Denervated Baroreceptors
Vagotornzed Post-Control
Pr-otw Pta-Control
Intravetwos stntrowr.-
Post-Control
Pte-Control
Intratrnol
Po Control
SW0 400 wooena
UnaY Ogmoaloty
(mOsr/kg H20
300
200 100
*60
Free-Water .40 Clearance (ml/min)
>1
.20
0-2£0
FIG. 2. Antidiuretic effect of intravenous isoproterenol in sham operated (left panel) and vagotomized (middle panel) dogs. Isoproterenol did not alter urinary osmolality or freewater clearance in animals with denervated arterial baroreceptors (right panel).
circulatory alteration was associated with a mean diminution in arterial pressure of approximately 25 mm Hg at the level of the baroreceptors and a consistent antidiuresis (Fig. 3). This antidiuretic effect was virtually abolished by prior removal of the pituitary source of vasopressin. In these studies an acute hypophysectomy via the buccal apptoach was performed prior to the experiments and the animals were replaced with glucocorticoid hormone. In these animals thoracic inferior vena caval constriction altered cardiac output and arterial pressure to a degree similar to that observed in the intact, control dogs. It was then examined whether this non-osmotic release of ADH was mediated by intact baroreceptors. The studies therefore were repeated in animals with an intact pituitary source of ADH release but with denervated arterial baroreceptors, i.e., bilateral section of both vagal and glossopharyngeal nerves. Acute lowering Of cardiac output and arterial pressure of thoracic caval constriction did not produce an antidiuresis in these experiments (Fig. 3). The results of these above series of experiments thus indicated that alteration in baroreceptor tone in the arterial vascular tree constitutes an important non-osmotic pathway for ADH release. Additional studies have demonstrated that even the antidiuretic effect of intravenous nicotine is dependent on intact parasympathetic pathways rather than exerting a direct effect on the hypothalamic release of ADH as previously suggested. 12
164
SCHRIER ET AL. Intact Neuro-Hypophyseal Tract
Hypophysectomized
Thso,ac .c Vena Ca ConstS, c ,on
Tho, &c c
L
I
Pre Control
1001
Pre Control
Urinary
,
Denervated Baroreceptors I
Posr Control
Pm Contl
I hor&C.c
IV.n. Ca..
I
Post Control
Consor.cton
/
600 Osmolal,ty 600 (mOsm kg H2O)
2001
Post Conrol
Infe,o, Vens C"av
lo Cos
iill
//_
/
km
i "N
Free Water 2 c Clearance (ml 'min) 2(
FIG. 3. Antidiuretic ef'fect of' acute thoracic inferior vena cava constriction on renal water excretion in intact dogs (left panel). Removal of the source of ADH release in the posterior pituitary (middle panel) or baroreceptor denervation (right panel) virtually abolishes this antidiuresis of thoracic caval constriction.
In view of' the previous studies suggesting the presence of' low pressure baroreceptors in the atrium, 13, 14 further studies were performed to examine whether increases in left atrial pressure during rapid atrial pacing were involved in the non-osmotic regulation of ADH release.15 Atrial pacing to increase heart rate by approximately 100 beats/min was f'ound to cause a consistent increase in renal water excretion (Fig. 4). This degree of atrial pacing raised mean left atrial pressure approximately 10 mm Hg in both intact and acutely hypophysectomized, steroid replaced animals. When the hypophysectomized animals received a constant infusion of' vasopressin throughout the experiments, atrial pacing did not increase renal water excretion. Moreover, bilateral cervical vagotomy also abolished this diuretic response to atrial pacing. Since the carotid sinus baroreceptors were intact and atrial pacing caused a modest (5-10 mm Hg) fall in arterial pressure (an effect which if anything would stimulate ADH release and cause an antidiuresis), the water diuresis observed with atrial pacing seemed to be mediated by alterations in parasympathetic afferent pathways from the low pressure system with subsequent suppression of' endogenous vasopressin release. In this regard, an increase in left atrial pressure by inflation of' an intra-atrial baloon also is associated with increased vagal tone and a water diuresis that is abolished by bilateral cervical vagotomy.16 Taken together, the results of the above studies indicate that the nonosmotic release of ADH is modulated by alterations in baroreceptor in both low (atrial) and high (arterial) pressure systems. In some
165
REGULATION OF RENAL WATER EXCRETION Intact Animals Pre Conlrol
1500
Atrial
Pacorg
Post Control
Hypophysectomized Animals Pre Control
Atrial
Pacing
Post Control
Animals with Cervical Vagotomy Pie Conrol
IPacing
Post Control
Urinary 1000 Osmolality
(m0sm/kg H20) 500
+1 0-
+0 5
Free-Water Clearance
OO
fcc/min) -0 5
-1 0
A
0__
..a
:.11.
FIG. 4. Diuretic effect of atrial pacing in dogs (left panel) and the effect of hypophysectomy (middle panel) and bilateral cervical vagotomy (right panel) to abolish this diuresis.
circumstances both low and high pressure baroreceptors may be activated, such as during an acute hemorrhage or an acute reduction in cardiac output. On the other hand, the high and low pressure baroreceptors may receive opposing stimuli, such as during atrial pacing. In this setting the left atrial receptors may prove to be more sensitive. However, with more rapid atrial pacing and larger decrements in arterial pressure, the high pressure baroreceptors may be more potent and predominate, thereby causing an antidiuresis. Circulatory disturbances which alter baroreceptor tone in the low or high pressure vascular system or both, thus provide important nonosmotic stimuli for ADH release. From a phylogenetic viewpoint, this non-osmotic release of ADH may constitute an integral part of the alarm reaction and in this setting the vascular effect of vasopressin may have been as important as its water conserving action, particularly in sea-f'aring species. 17 While the release of ADH seems to constitute an important nonosmotic factor inf'luencing renal water excretion, there are also intrarenal f'actors which are important determinants of' renal water excretion, including renal arterial pressure, glomerular filtration rate, renal blood
166
SCHRIER ET AL.
flow and solute excretion. In addition to these factors we have examined in vivo the role of renal prostaglandins in modulating the hydrosmotic effect of vasopressin. Previous in vitro experiments in anuran membranes had suggested that prostaglandin of the E series impair the effect of vasopressin to both increase cyclic AMP and enhance osmotic water movement. 18-21 In the present experiments two sequential 100 mU boluses of vasopressin were injected into anesthetized dogs undergoing a water diuresis. Between the two boluses of vasopressin either a blank soluition or an inhibitor of prostaglandin synthesis (indomethacin or meclofenamate) was administered.22 As shown in Figure 5, the repeated doses of vasopressin, with the blank solution intervening, produced a comparable antidiuresis; but the administration of either m-eclofenamate or indomethacin markedly enhanced the antidiuretic effect of vasopressin. The results were compatible with the interpretation that renal medullary prostaglandins do inhibit the hydrosmotic action of vasopressin. This effect of prostaglandins could be mediated by regulating medullary blood flow, and thus influencing the rate of medullary washout of the hypertonic interstitium, or by impairing the vasopressinmediated generation of cyclic AMP. In regards to the latter possibility, it is important to emphasize that medullary collecting duct cells have been shown by histochemical techniques to contain prostaglandin.23 Several other factors also favor the mechanism involving cyclic AMP. One of us 41
Urinary
Osmolaity
(rnOsm/kg H20)
* Blank Solution *.. Indomnethacen 0---0 Meclofenamate
FIG. 5. Effect of inhibition of prostaglandin synthesis with indomethacin or meclofenamate to enhance the antidiuretic effect of vasopressin.
o
REGULATION OF RENAL WATER EXCRETION
I
1.
21,
u CL
14 p
