Effects of pertussis and cholera toxins on alpha-adrenoceptor function in rat tail artery: differences in hypertension.

The alpha 1- and alpha 2-adrenoceptor-stimulated contractile responses of rat tail artery rings were compared in Sprague-Dawley (SD), spontaneously hypertensive (SHR), and Wistar-Kyoto (WKY) rats that were untreated, treated with pertussis toxin, or treated with cholera toxin. The maximal responses, expressed as milligrams of tension, induced by clonidine (an alpha 2-adrenoceptor agonist) and cirazoline (a selective alpha 1-adrenoceptor agonist) were significantly greater in SHR than in SD or WKY, and the tissues were more sensitive to the agonists in SHR or SD than in WKY. Yohimbine (0.1 microM), a selective alpha 2-adrenoceptor antagonist, shifted the dose-response curves for clonidine to the right. The effects of yohimbine were greater in SD than in WKY or SHR, but not different between WKY and SHR. Prazosin (0.05 microM), a selective alpha 1-adrenoceptor antagonist, shifted the dose-response curves of cirazoline to the right, but the effects of prazosin were not different among these three strains of rats. Nifedipine (0.05 microM) completely blocked the response to clonidine in SD and WKY; however, in SHR, approximately one-third of the response to clonidine was resistant to nifedipine. Nifedipine, at 0.05 microM, only partially inhibited responses to cirazoline in SD, SHR, and WKY, and no differences were noted between the strains. Pertussis toxin pretreatment (50 micrograms/kg, 3 days before experiment) almost completely blocked the responses to clonidine, but only partially inhibited those to cirazoline. After pertussis toxin pretreatment, the responses (maximal effects and EC50s) to clonidine and cirazoline were not significantly different in arteries from the three strains of rats.(ABSTRACT TRUNCATED AT 250 WORDS)