ALPHA-ADRENERGIC REACTIVITY OF THE MICROCIRCULATION IN CONSCIOUS SPONTANEOUSLY HYPERTENSIVE RATS

The goal of this study was to determine the functional distribution of alpha(1)- and alpha(2)-adrenoceptors in the striated muscle microcirc ulation. Experiments were performed in intact conscious spontaneously hypertensive rats (SHR) that were provided with a dorsal microcirculat ory chamber to allow microvascular diameter measurements. Administrati on of selective alpha(1)- and alpha(2)-agonists, phenylephrine and aze pexole, respectively, induced different patterns of microvascular cons triction. alpha(1)-Adrenoceptor stimulation showed a preferential cons triction of large arteries and venules. The entire arteriolar microvas culature was sensitive to alpha(2)-adrenoceptor stimulation, whereas t he venular vessels did not respond to azepexole. The selective alpha(1 )- and alpha(2)-antagonists prazosin and yohimbine showed patterns of vasodilator activity comparable to those of the corresponding agonists . The specificity of the drug-induced effects was verified by comparin g their effects with those of graded hemorrhage, a non-pharmacological method for blood pressure lowering. In the range of blood pressure de creases comparable to that obtained by alpha-adrenoceptor antagonists, graded hemorrhage did not influence microvascular diameters. These re sults show a differential functional distribution of alpha(1)- and alp ha(2)-adrenoceptors along the microvascular tree in striated muscle of conscious SHR.