RECIPROCAL INHIBITION OF CARDIAC RESPONSES TO NORE-PINEPHRINE AND INSULIN

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The effects of insulin (IN) and norepinephrine (NE) on left ventricular contractility (LVc) were studied in 20 piglets. Aortic pressure, cardiac output, heart rate and temperature were controlled, and LV dP/dt max was taken to reflect LVc. Hypoglycemia was prevented by giving glucose as necessary. LVc increased 28% (p<.02) 45 min. after IN (20U). Following β-blockade (practolol, 1 mg/kg), in caused a 55% increase in LVc. NE dose-response (DR) curves were obtained before and after IN. Maximal changes in LVc with each dose of NE (0.5-2.0μg) were significantly less in hearts exposed to IN. Duplicate DR curves were identical, indicating that prior exposure to NE did not inhibit subsequent responses. However, in those piglets initially subjected to a NE dose-response curve, IN elicited only an 11% increase of LVc, and this was not significant. Thus, the inotropic response was attenuated (p<.01) in animals previously exposed to NE, Conversely, in animals first subjected to β-blockade, the response to IN was greater (p<.005). These data indicate a reciprocal blocking action by these hormones on piglet myocardium. The cellular mechanisms responsible for these findings remain to be elucidated, but likely involve modulation of Ca++ fluxes.