Selective interaction of alpha-adrenoceptors with myogenic regulation of microvascular smooth muscle.

The interaction of alpha 1- and alpha 2-adrenoceptor constriction of arterioles with the myogenic mechanism was examined in rat cremaster skeletal muscle using intravital microscopy. Intravascular pressure was increased or decreased by changing pressure from 0 to +30 and 0 to -20 mmHg in a chamber that surrounded the animal but not the cremaster. Arterioles of 110 microns in diameter were constricted by approximately 30% with the alpha 1-agonist, phenylephrine, or the alpha 2-agonist, UK-14304. Increases in microvascular (box) pressure caused stepwise constrictions of similar magnitude (10-30%) regardless of the prevailing type of alpha-adrenergic tone. In contrast, during alpha 2 tone, decreases in pressure caused a three- to fourfold greater dilation (myogenic inhibition) than during alpha 1 tone. Thus myogenic constriction, normally minimal in these large arterioles, was amplified similarly during either alpha 1 or alpha 2 tone; however, alpha 2 tone was much more susceptible than alpha 1 tone to myogenic inhibition. Similar results were obtained with tone produced by thromboxane A2 vs. the Ca2+ channel agonist, BAY K 8644. Thus the particular sensitivity of alpha 2 constriction to myogenic inhibition may relate to interactions between these stimuli at the postreceptor level. The degree and type of alpha-adrenoceptor tone at different microvasculature levels may be important in myogenic autoregulatory blood flow adjustments during changes in perfusion pressure.