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Spectrophotometric study of alpha-adrenoceptors affecting microcirculation of rat skin.

1. To determine the alpha-adrenergic receptor subtypes that affect the microcirculation of skin, the relative absorption (RA) spectra of the skin on the backs of rats were measured using reflectance spectrophotometric methods. We injected alpha-adrenergic agonists, noradrenaline (NA), phenylephrine (PE) and clonidine (CL), intravenously and determined changes in the RA value at 569 nm, one of the isosbestic points of the oxyhaemoglobin and deoxyhaemoglobin absorption. 2. NA reduced the RA value and the reduction was inhibited significantly by pretreatment with phenoxybenzamine (PBZ; P < 0.01). These findings suggested that the haemoglobin content in the skin tissue decreased as a result of vasoconstriction through alpha-adrenoceptors. 3. NA, PE and CL produced dose-dependent reductions in RA. CL and NA produced virtually equipotent reductions except at the highest dose used. PE produced smaller effects. The potency of these drugs in terms of changes in RA did not correlate with their potency in terms of rises in systemic blood pressure (NA > PE > or = CL). 4. Yohimbine (YO) inhibited the NA-induced reduction in RA to a greater degree than bunazosin (BU). Midaglizole, a specific alpha 2-adrenergic antagonist, significantly and dose dependently inhibited the NA-induced reduction in RA. 5. Although BU inhibited NA-induced reduction in RA only slightly, the effect was significant (P < 0.05). BU significantly inhibited PE-induced reduction (P < 0.01), but did not inhibit CL-induced reduction. 6. These observations suggest that the microcirculation of the skin of the rat is affected mainly by alpha 2-adrenoceptor mediated vasoconstriction. However, alpha 1-adrenoceptor mediated vasoconstriction also has some effect.