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 spectrophot ometric methods. We injected alpha-adrenergic agonists, noradrenaline (NA), phenylephrine (PE) and clonidine (CL), intravenously and determi ned 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 pretreatmen t with phenoxybenzamine (PBZ; P < 0.01). These findings suggested that the haemoglobin content in the skin tissue decreased as a result of v asoconstriction through alpha-adrenoceptors. 3. NA, PE and CL produced dose-dependent reductions in RA. CL and NA produced virtually equipot ent reductions except at the highest dose used. PE produced smaller ef fects. The potency of these drugs in terms of changes in RA did not co rrelate with their potency in terms of rises in systemic blood pressur e (NA > PE greater-than-or-equal-to CL). 4. Yohimbine (YO) inhibited t he NA-induced reduction in RA to a greater degree than bunazosin (BU). Midaglizole, a specific alpha2-adrenergic antagonist, significantly a nd dose dependently inhibited the NA-induced reduction in RA. 5. Altho ugh BU inhibited NA-induced reduction in RA only slightly, the effect was significant (P < 0.05). BU significantly inhibited PE-induced redu ction (P < 0.01), but did not inhibit CL-induced reduction. 6. These o bservations suggest that the microcirculation of the skin of the rat i s affected mainly by alpha2-adrenoceptor mediated vasoconstriction. Ho wever, alpha1-adrenoceptor mediated vasoconstriction also has some eff ect.