Effects of Norepinephrine in the Presence of Adrenergic Antagonists and Different Ionic Media upon the Mechanical and 86Rubidium Efflux Responses in Vascular Smooth Muscle

Abstract

The action of norepinephrine (NE) on the isolated guinea pig mesenteric–portal vein was studied in relation to its mechanical and 86rubidium (86Rb) efflux responses using a continuous flow technique. NE (1 × 10−8 to 2 × 10−6 g/ml) produced dose-dependent increases in both contraction and 86Rb efflux. Phentolamine but not propranolol inhibited both responses to NE. In high potassium medium the 86Rb efflux response was more markedly depressed than the mechanical response. In calcium-deficient medium the NE-induced contraction was abolished but the 86Rb efflux response was potentiated and in high calcium medium there was a significant depression of the mechanical response but no significant difference in the 86Rb efflux response. There seems to be an optimal calcium ion concentration at 2.5 mM for the mechanical responses to NE. These experiments suggest that contraction correlates strongly with an increase in membrane permeability, calcium is required for vascular muscle contraction, and contraction can be initiated by a mechanism independent of changes in membrane potential.