INTRODUCTION

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IN ADDITION to the well-known actions of oxytocin (OT) during lactation and parturition, OT is a natriuretic hormone (25). Indeed, neurohypophysial OT secreted in response to osmotic stimulation in rats has been documented to contribute importantly to the natriuresis observed under these conditions (5, 7). OT is also secreted in large amounts in response to hypotension (14) or hypovolemia (20), although its actions under these conditions remain obscure.

Binding sites for OT exist in the macula densa (19). The macula densa is known to stimulate renin secretion (18), which contributes importantly to cardiovascular homeostasis during hypotension or hypovolemia. In recent studies we noted that intravenous infusion of OT in physiological doses stimulates renin secretion in anesthetized rats and that the action of OT on renin release is not secondary to its natriuretic effects (17). The present studies sought to determine whether infusion of OT increases plasma renin levels in conscious rats. Because the results indicated that infusion of OT did increase plasma renin levels, additional studies were conducted to determine whether this response required -adrenoceptor-dependent mechanisms, which would suggest an action independent of the macula densa.

	METHODS

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Animals. Adult male Sprague-Dawley rats (Zivic Laboratories, Zeleinople, PA), weighing 350-400 g, were housed individually in wire-mesh cages in a colony room with ambient temperature maintained at 22-24°C and with lights on from 8 AM to 8 PM. Rats had ad libitum access to Purina Laboratory Chow pellets and tap water.

Experimental protocols. One day before the experiments, all rats were anesthetized with Equithesin (3.0 ml/kg body wt ip), a solution containing pentobarbital sodium (0.98 g/dl), chloral hydrate (4.25 g/dl), and MgSO₄ (2.12 g/dl). Catheters were placed into the right femoral artery and the right femoral vein. The free ends of the two catheters were guided subcutaneously along the back to exit between the scapulae. On exiting, the catheters were encased in a steel spring to prevent them from being damaged. Rats were returned to their home cages, with the catheters leaving the cages to make them accessible without disturbing the rats.

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Analysis of plasma renin. All blood samples were withdrawn from the arterial catheters into tubes coated with 3.7 mg of potassium EDTA and bathed in ice. Blood samples were centrifuged (1,100 g for 10 min), and the plasma was removed and stored at 80°C.

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Statistics. Values are means ± SE. Data were analyzed by two-way (group × time) ANOVA (Systat, Evanston, IL) with repeated measures in the time parameter. The error terms and degrees of freedom from the ANOVA were used in t-tests to compare treatment values with baseline values within groups. Comparisons between groups at specific time points were done using Tukey's honestly significant difference test. P < 0.05 was considered to be statistically significant.

	RESULTS

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OT infused at 25 ng · kg¹ · h¹ iv had no effect on PRC in conscious rats (Fig. 1). However, PRC was increased more than twofold by infusions of OT at 125 ng · kg¹ · h¹ (P < 0.05; Fig. 1). Infusion of OT at these rates altered neither MAP nor HR at any time during the infusion (data not shown).

View larger version (20K): [in this window] [in a new window]   Fig. 1.   Effect of intravenous infusion of oxytocin (OT) on plasma renin concentration (PRC). One group of rats (n = 7) was infused with OT at 25 ng · kg1 · h1 for 1 h, then at 125 ng · kg1 · h1 for 1 h. Control rats (n = 7) were infused with isotonic saline (5 ml · kg1 · h1) during this 2-h period. Blood samples for measurement of PRC were collected just before start of infusion period (time 0) and at 20 and 60 min of 1st and 2nd h of infusion. OT infused at 125 ng · kg1 · h1 significantly increased PRC compared with baseline values or control rats (both P < 0.05).

In a separate group of rats, infusion of OT at 125 ng · kg¹ · h¹ again increased PRA by twofold (P < 0.01; Fig. 2). Pretreatment with an OT receptor antagonist did not alter baseline PRA but completely prevented the OT-induced increase in PRA (Fig. 2). The -adrenergic receptor antagonist nadolol (2.5 mg/kg iv) injected 15 min before infusion of OT slightly reduced baseline PRA and appeared to completely prevent the effects of OT on renin secretion (Fig. 2). Similarly, nadolol injected after 60 min of OT infusion abruptly reduced PRA (P < 0.05; Fig. 2). Neither OT nor the OT antagonist caused significant changes in MAP or HR (Table 1). Nadolol did not change MAP but reduced HR by 20-40 beats/min whether it was given before or during infusion of OT (P < 0.05; Table 1).

View larger version (19K): [in this window] [in a new window]   Fig. 2.   Effect of OT antagonist (OT-ant) or nadolol on OT-stimulated renin secretion. After collection of baseline blood samples for measurement of plasma renin activity (PRA), groups of rats (n = 7 or 8) were pretreated with OT antagonist, nadolol, or isotonic saline, and a 2nd blood sample was collected. Then all rats were infused with OT at 125 ng · kg1 · h1, and blood samples were collected after 30 and 60 min of infusion. After 60 min of OT infusion, vehicle-pretreated rats were injected with nadolol (arrow), and an additional blood sample was taken 15 min later. OT significantly increased PRA from baseline only in the group pretreated with vehicle (P < 0.01). Nadolol also significantly reduced PRA when administered during OT infusion (P < 0.01).

In contrast to the blocking effect of an OT receptor antagonist on OT-induced renin secretion (Fig. 2), renin release evoked by injection of the -adrenergic receptor agonist isoproterenol (10 µg/kg) was not altered by the OT antagonist (Fig. 3). The OT receptor antagonist also did not alter the decrease in MAP or the increase in HR caused by isoproterenol (Table 2).

View larger version (20K): [in this window] [in a new window]   Fig. 3.   Effect of OT antagonist on renin secretion induced by isoproterenol (Iso). Groups of rats (n = 7 or 8) were infused with OT antagonist or vehicle (Veh) for 1 h and then injected with isoproterenol (10 µg/kg iv; arrow). Blood samples for measurement of PRA were collected at beginning of experiment (baseline), just before injection of isoproterenol (time 0), and 5, 15, and 30 min after injection of isoproterenol. Isoproterenol produced a significant increase in PRA (P < 0.05), which was similar whether rats were pretreated with OT antagonist or vehicle.

	DISCUSSION

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The major finding of the present study is that plasma renin levels were increased by intravenous infusion of OT in a physiological dose in conscious, freely moving rats. Furthermore, this substantial increase in plasma renin levels produced by OT was prevented by pretreatment with an OT receptor antagonist or with a -adrenergic receptor antagonist.

OT was infused at 125 ng · kg¹ · h¹ to simulate the increased plasma levels of OT measured during hypotension or hypovolemia (14, 20). Infusion of OT at this rate resulted in a doubling of PRC and PRA. Similar results were observed previously using thiobutabarbital (Inactin)-anesthetized rats (17). In contrast, infusion of OT at 25 ng · kg¹ · h¹, a dose selected to mimic the smaller increase in plasma OT levels caused by 24 h of water deprivation (5), did not significantly alter PRC in conscious rats. These observations allow the possibility that the high plasma OT levels observed during hypotension and hypovolemia may promote renin secretion and thereby make a useful contribution to the support of blood pressure, as does neurohypophysial vasopressin [which also is secreted under these conditions (14, 20)].

OT likely evokes renin release via its action on OT receptors, because this effect was prevented by pretreatment with a selective OT receptor antagonist. The antagonist used in this study has been shown previously to block the natriuretic effects of OT but not to interfere with vasopressin receptors (6). The specificity of this antagonist for renin release induced by OT was suggested by the observation that it did not block renin secretion induced by isoproterenol.

Although these studies were originally prompted by the observation that OT binding sites are present in the macula densa, the macula densa is probably not the site at which OT acts to elicit renin secretion in the present experiments. This view is based on observations that renin secretion stimulated by the macula densa is independent of -adrenergic receptor stimulation (9, 15), whereas in the present study OT-induced renin secretion was largely attenuated, if not prevented, by injection of nadolol. This effect of nadolol suggests that OT acts to increase renal sympathetic nerve activity or adrenal medullary catecholamine secretion (8). Further studies are needed to determine whether OT acts on renal sympathetic nerve terminals, on sympathetic ganglia, directly in the central nervous system to increase sympathoadrenal outflow, or on afferent nerves to reflexively elicit this response. Responses mediated by -adrenergic receptors independent of the sympathoadrenal system are also a possibility (11, 23). In this regard, a previous study (2) in which renin secretion was stimulated by infusion of OT into the vertebral artery of anesthetized dogs points to the brain as a likely site of action. Although OT is unlikely to penetrate the blood-brain barrier, its action on a circumventricular organ is possible.

The general hypothesis that a circulating factor may influence renin secretion via -adrenergic receptors is consistent with previous reports (8, 23, 24). Adrenomedullary secretions during stress are well known to stimulate renin secretion, but they may not be the only blood-borne factors to do so (1, 8). Morton et al. (11) and Van de Kar and Richardson-Morton (23) reported that serotonin agonists cause renin release in rats that is blocked by -adrenergic receptor antagonists but does not require intact renal sympathetic outflow. Many of the features of their blood-borne renin-releasing factor are consistent with the active agent being OT; like OT, it is a 1,000- to 5,000-Da peptide present in the hypothalamus (12, 22, 24). Furthermore, serotonin agonists are known to cause pituitary OT release (13). Although Van de Kar et al. (24) failed to find renin-releasing activity in the pituitary gland, it is possible that such activity was obscured by the high concentration of vasopressin, which is known to inhibit renin secretion (4). De Vito et al. (3) also presented evidence for a blood-borne renin-releasing factor that appeared in the circulation in response to hypotension in dogs.

In summary, the present results indicate that renin secretion is stimulated by increases in plasma OT similar to those produced physiologically by hypotension or hypovolemia. This finding raises the possibility that renin secretion elicited in response to OT contributes to the homeostatic response to such cardiovascular challenges. Indeed, we recently noted that renin release in response to hydralazine-induced hypotension is markedly attenuated by systemic infusion of an OT receptor antagonist (21). Further work is needed to confirm those observations and to elucidate the mechanisms by which OT exerts this effect.

Perspectives