We have previously shown that oxytocin receptors are present in the heart a
nd that perfusion of isolated rat hearts with oxytocin results in decreased
cardiac flow rate and bradycardia. The mechanisms involved in the negative
inotropic and chronotropic effects of oxytocin were investigated in isolat
ed dog right atria in the absence of central mechanisms. Perfusion of atria
through the sinus node artery with 10(-6) mol/L oxytocin over 5 minutes (8
mL/min) significantly decreased both beating rate (-14.7 +/-4.9% of basal
levels, n=5, P<0.004) and force of contraction (-52.4<plus/minus>9.1% of ba
sal levels, n=5, P<0.001). Co-perfusion with 10(-6) mol/L oxytocin receptor
antagonist (n=3) completely inhibited the effects of oxytocin on frequency
(P<0.04) and force of contraction (P<0.004), indicating receptor specifici
ty. The effects of oxytocin were also totally inhibited by co-perfusion wit
h 5 X 10(-8) mol/L tetrodotoxin (P<0.02) or 10(-6) mol/L atropine (P<0.03)
but not by 10(-6) mol/L hexamethonium, which implies that these effects are
neurally mediated, primarily by intrinsic parasympathetic postganglionic n
eurons. Co-perfusion with 10(-6) mol/L NO synthase inhibitor (L-NAME) signi
ficantly inhibited oxytocin effects on both beating rate (-1.85<plus/minus>
1.27% versus -14.7 +/-4.9% in oxytocin alone, P<0.05) and force of contract
ion (-24.9<plus/minus>4.4% versus -52.4 +/-9.1% in oxytocin alone, n=4, P<0
.04). The effect of oxytocin on contractility was further inhibited by L-NA
ME at 10(-4) mol/L (-8.1<plus/minus>1.8%, P<0.01). These studies imply that
the negative inotropic and chronotropic effects of oxytocin are mediated b
y cardiac oxytocin receptors and that intrinsic cardiac cholinergic neurons
and NO are involved in these actions.