Modulation by the nutritional state of adrenergic receptor mediated effects on insulin secretion in ducks.

Abstract

The effects of exogenous (IV Norepinephrine (NE)) and endogenous alpha adrenergic activity (IV phentolamine (PH)) were studied in the duck. Our aim was first to compare their importance in animals fed for 1 1/2 hours and fasted for 24 and 48 hours, and then to assess the relative importance of alpha versus beta adrenergic receptor mediated effects during the development of a 48-hour-fast. NE reduces plasma IRI levels after a fast but not in the postprandial state. PH while ineffective in animals fed for 1 1/2 hours brings fasting IRI values to levels observed in the former group. Twenty-four hour fasted ducks are sensitive to both alpha adrenergic stimulation and beta adrenergic blockade which have additive effects; so, insulin secretion is controlled by 2 distinct mechanisms: a beta stimulating and an alpha inhibiting one. The importance of one component versus the other is modulated by the nutritional state: indeed, a double alpha and beta equimolar adrenergic blockade reveals that during the transition from the postprandial state to a 48-hour-fast, there is a progressive reversal in the importance of one adrenergic component versus the other. The effects of alpha adrenergic stimulation or blockade occur independently of simultaneous glucose variations. This is not the case for A cell function: an alpha adrenergic receptor stimulating effect on glucagon secretion exists, but A cell remains primarily sensitive to plasma glucose level. alpha Adrenergic activity has no direct effect on plasma free fatty acids (FFA) level but plays a role in plasma glucose level regulation especially in animals in the postprandial state.