EFFECT OF COOLING ON CUTANEOUS MICROVASCULAR ADRENOCEPTORS IN-VIVO INTHE RABBIT EAR

Previous studies have suggested that moderate cooling increases the re sponsiveness of vascular alpha(2)-adrenoceptors. However, limited info rmation is available documenting the influence of temperature changes on adrenoceptor responses in the microvasculature of thermoregulatory organs (e.g., the human digit and the rabbit ear) subjected to a wide range of temperatures. In the present study, the effect of local cooli ng (24 degrees C) on cutaneous microvascular adrenoceptors in the ear was observed in vivo in male New Zealand White rabbits (total: 66 ears ). The rabbit ear was studied in a temperature-controlled tissue bath; the ear preparation was pretreated with terazosin (an alpha(1)-adreno ceptor antagonist) (10(-5) M) or a combination of terazosin (10(-5) M) and propranolol (a beta-adrenoceptor antagonist) (10(-6) M). The micr ovascular diameter responses of the ear to norepinephrine (10(-11)-10( -4) M) then were determined at 24 or 34 degrees C, respectively, to de termine the influences of low temperature on adrenoceptor responses to norepinephrine stimulation. The results demonstrated that low concent rations of norepinephrine induced vasodilation in arterioles and arter iovenous anastomoses. This vasodilation was followed by vasoconstricti on with an increased concentration of norepinephrine in animals with a lpha(1)-adrenergic blockade at 34 degrees C. Moderate tissue cooling i ncreased the microvascular maximal response of the rabbit ear to norep inephrine and abolished the vasodilatation induced by a low concentrat ion of norepinephrine. There was no significant difference in the micr ovascular response to norepinephrine between the two temperature condi tions after simultaneous blockade of alpha(1)-adrenoceptors and beta-a drenoceptors, Data from the present study indicate that moderate cooli ng does not enhance the responsiveness of alpha(2)-adrenoceptors to no repinephrine. In contrast, cooling reduced the beta-adrenergic activit y of arterioles and arteriovenous anastomoses after norepinephrine sti mulation.