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 microcirculation. Experiments were performed in intact conscious spontaneously hypertensive rats (SHR) that were provided with a dorsal microcirculatory chamber to allow microvascular diameter measurements. Administration of selective alpha(1)- and alpha(2)-agonists, phenylephrine and azepexole, respectively, induced different patterns of microvascular constriction. alpha(1)-Adrenoceptor stimulation showed a preferential constriction of large arteries and venules. The entire arteriolar microvasculature was sensitive to alpha(2)-adrenoceptor stimulation, whereas the 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 comparing their effects with those of graded hemorrhage, a non-pharmacological method for blood pressure lowering. In the range of blood pressure decreases comparable to that obtained by alpha-adrenoceptor antagonists, graded hemorrhage did not influence microvascular diameters. These results show a differential functional distribution of alpha(1)- and alpha(2)-adrenoceptors along the microvascular tree in striated muscle of conscious SHR.