Abstract
The patch clamp technique was used to make whole-cell recordings of calcium channel currents from single muscle cells freshly isolated from rat mesenteric arteries. The cells were found to contain two types of calcium channels; one type is activated by small depolarizations and inactivates quickly, whereas the other requires stronger depolarizations for activation and inactivates more slowly. Nitrendipine blocked the second type of channels with a potency that depended on membrane potential. Interpreted by a modulated receptor hypothesis, these results suggest that nitrendipine binds to the inactivated state of the channel with a KD of about 0.5 nM, similar to concentrations effective in relaxing blood vessels. The magnitude of the calcium channel current was small compared with other excitable cells, but the "calcium agonist" drug BAY K8644 produced a 10-fold augmentation of calcium channel current, suggesting the existence of a large "reserve" of calcium channel current.
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