Increased calcium sensitivity in isolated resistance arteries from spontaneously hypertensive rats: effects of dihydropyridines.

We recorded the contractile responses to calcium in mesenteric resistance arteries of Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) during depolarization or stimulation with noradrenaline. The effects of Bay-K8644 and nimodipine on these responses were evaluated. Calcium sensitivity was greater in noradrenaline-stimulated than in depolarized vessels. Nimodipine decreased and Bay-K8644 increased calcium sensitivity. Both substances were more potent in the presence of potassium than in the presence of noradrenaline. Calcium sensitivity was greater in SHR than in WKY vessels only during stimulation with noradrenaline. The rhythmic responses of SHR vessels during stimulation with noradrenaline were abolished by nimodipine. Rhythmicity could be induced in WKY vessels by Bay-K8644. Modulation of calcium sensitivity by dihydropyridines during electrochemical as well as pharmacological stimulation suggests that, in resistance arterial smooth muscle, the function of potential-operated calcium channels can be modulated by noradrenaline. This modulation could differ quantitatively between mesenteric resistance arteries of SHR and WKY.