Effect of temperature on alpha-adrenoceptor affinity and contractility of rabbit ear blood vessels

We have studied contractile responses to norepinephrine (NE) and K+ of isolated rabbit ear arteries and veins in an effort to determine how alpha-adrenoceptor affinity and smooth muscle contractility affect responsiveness at different temperatures. Arteries have predominantly alpha 1-adrenoceptors and veins have predominantly receptors of the alpha 2-type. We obtained dose-response curves to NE at 9, 16, 23, 30, 37 and 42 degrees C in the absence and presence of the irreversible alpha-adrenoceptor blocker phenoxybenzamine (POB). In both vessels, affinity of the alpha-adrenoceptors was determined by comparing equieffective doses of NE before and after blockade of the receptors. In other experiments, we obtained contractions to 80 mM KCl at the same temperatures. Affinity of the ear arterial alpha 1-receptors was maximal at 23 degrees C, while affinity of ear venous alpha 2-receptors increased linearly with cooling to 9 degrees C. Responses to K+ in both vessels decreased with cooling. Taken together, the changes in receptor affinity and response to K+ may explain the shape of curves relating contractile strength to temperature.