Acute hypotension alters hemodynamic response to methionine-enkephalin in conscious dogs

In the conscious dog, intravenous administration of methionine-enkephalin produces simultaneous increases in both heart rate (HR) and mean arterial blood pressure (MAP). This report describes both depressor and cardioaccelerator responses to methionine-enkephalin (10 micrograms/kg IV) in conscious dogs following acute hypotension induced by either bolus injection of isoproterenol (0.1-5.0 micrograms/kg IV) or infusion of sodium nitroprusside (SNP, 3-8 micrograms/kg/min). Cardiovascular responses to methionine-enkephalin were blocked by naloxone. Pretreatment of the dogs with the beta-adrenergic receptor antagonist propranolol failed to prevent the hypotensive response to methionine-enkephalin following SNP infusion. The results indicate that the hemodynamic responses to methionine-enkephalin can be altered by acute manipulation of blood pressure. These results may have implications relative to the role of endogenous opiates in regulation of blood pressure, especially in acute hypotensive states.