Intrathecal injection of an oxytocin-receptor antagonist attenuated postnephrectomy natriuresis in the male rat

Oxytocinergic regulation of cardiovascular function: studies in oxytocin-deficient mice

Oxytocin (OT) has been implicated in the cardiovascular responses to exercise, stress, and baroreflex adjustments. Studies were conducted to determine the effect of genetic manipulation of the OT gene on blood pressure (BP), heart rate (HR), and autonomic/baroreflex function. OT knockout (OTKO -/-) and control +/+ mice were prepared with chronic arterial catheters. OTKO -/- mice exhibited a mild hypotension (102 +/- 3 vs. 110 +/- 3 mmHg). Sympathetic and vagal tone were tested using beta(1)-adrenergic and cholinergic blockade (atenolol and atropine). Magnitude of sympathetic and vagal tone to the heart and periphery was not significantly different between groups. However, there was an upward shift of sympathetic tone to higher HR values in OTKO -/- mice. This displacement combined with unchanged basal HR led to larger responses to cholinergic blockade (+77 +/- 25 vs. +5 +/- 15 beats/min, OTKO -/- vs. control +/+ group). There was also an increase in baroreflex gain (-13.1 +/- 2.5 vs. -4.1 +/- 1.2 beats x min(-1) x mmHg(-1), OTKO -/- vs. control +/+ group) over a smaller BP range. Results show that OTKO -/- mice are characterized by 1) hypotension, suggesting that OT is involved in tonic BP maintenance; 2) enhanced baroreflex gain over a small BP range, suggesting that OT extends the functional range of arterial baroreceptor reflex; and 3) shift in autonomic balance, indicating that OT reduces the sympathetic reserve.

Prevention of reflex natriuresis after acute unilateral nephrectomy by melanocortin receptor antagonists

gamma-Melanocyte-stimulating hormone (gamma-MSH), atrial natriuretic peptide (ANP), and oxytocin have been identified as candidate hormonal mediators of the reflex natriuresis that follows acute unilateral nephrectomy (AUN). Pharmacological characterization of the third melanocortin receptor (MC3-R) indicates that it uniquely responds to physiological concentrations of gamma-MSH. We tested the roles of gamma-MSH, ANP, and oxytocin in the postnephrectomy natriuresis by carrying out AUN during continuous intrarenal infusion of specific antagonists for their cognate receptors. In anesthetized Sprague-Dawley rats, urinary sodium excretion (UNaV) increased from 0.34 +/- 0.04 to 1.12 +/- 0.11 mu eq/min 90 min after AUN (P < 0.001). No change in UNaV occurred in rats undergoing a sham AUN procedure. Plasma immunoreactive gamma-MSH concentration was 53 +/- 8 fmol/ml after sham AUN but 112 +/- 17 fmol/ml after AUN (P < 0.01). SHU-9119 and SHU-9005 are substituted derivatives of alpha-MSH with potent antagonism at the MC3-R in vitro. Infusion of these compounds at 5 pmol/min completely blocked the natriuretic response to AUN despite a similar elevation in plasma gamma-MSH (111 +/- 12 vs. 49 +/- 8 fmol/ml in sham rats, P < 0.01). Intrarenal infusion of the ANP receptor antagonist A-71915 (5 pmol/min) or the oxytocin receptor antagonist [d(CH2)(5)1, Tyr(Me)2,Orn8] vasotocin (10 pmol/min) effectively inhibited the natriuresis induced by intravenous infusion of ANP or oxytocin (each at 1 pmol/min), respectively, but did not block the natriuresis after AUN. Plasma immunoreactivity of these peptides was not increased after AUN. These results indicate that reflex natriuresis after AUN is accompanied by an increase in plasma gamma-MSH but not ANP or oxytocin concentration and is prevented by intrarenal infusion of receptor antagonists with selectivity for MC3-R. The data indicate that gamma-MSH or a closely related peptide mediates postnephrectomy natriuresis and provide further support for the possibility that gamma-MSH may play a wider role in sodium homeostasis.