Abstract
1. The calcium channel activator Bay K8644 increased the potency of noradrenaline in cat middle cerebral ( α 2-adrenoceptors) and mesenteric (atypical or mixed α 1- and α 2-adrenoceptor population) arteries, but not in rat middle cerebral and mesenteric arteries ( α 1-adrenoceptors). 2. In cat arteries, exposure to 15mM K + solution shifted the noradrenaline concentration-response curve to the left in an almost identical manner as did Bay K8644. 3. Bay K8644 completely reversed the relaxation produced by nifedipine in K +-contracted cat middle cerebral arteries, whereas the relaxation induced by verapamil, diltiazem or flunarizine was only partially reversed. This suggests a specific interaction between Bay K8644 and the dihydropyridine receptors on the calcium channels. 4. It is concluded that the degree to which noradrenaline promotes calcium influx through membrane channels is at least partly related to the α-adrenoceptor subtype mediating the response.
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