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Department of Pharmacology, St George's Hospital Medical School, London.

1. Ca2+ inward current was studied using the whole-cell patch clamp technique in single smooth muscle cells enzymatically isolated from the rabbit ear artery. Currents were studied in salt solutions containing either normal (1.5 mM) Ca2+ or high (110 mM) Ba2+. Outward currents were minimized by using a high-Na+ intracellular solution containing 10 mM-TEA. 2. In normal-Ca2+ solution, the threshold at which inward current could be evoked was -60 mV at a holding potential of -80 mV and -48 mV at a holding potential of -60 mV. At both holding potentials, the current showed little inactivation over 500 ms near threshold, and inactivated substantially but incompletely with larger depolarizations. In high-Ba2+ solution where currents were 5-10 times larger, current threshold, maximal peak amplitude and apparent reversal potential were shifted in a positive direction along the voltage axis. 3. Inward current in normal-Ca2+ solution was only fully available negative to -90 mV, was half-inactivated near -47 mV and was about 90% inactivated at -10 mV. A component of non-inactivating current was, however, present in both normal-Ca2+ and high-Ba2+ solutions even after conditioning pulses of 5 s duration to +30 mV. The inactivation-potential relationship was shifted in a positive direction in high-Ba2+ solution, and its position showed considerable variation between cells. 4. The inward current from a holding potential of -70 mV was reduced in normal-Ca2+ solution by about 50% by nifedipine (0.1 microM) in some cells at all test potentials, although at more-negative test potentials peak current was unaffected in some cells even at 10 microM. In some cells when the holding potential was -90 mV, the peak current amplitude was increased markedly by 10 microM-nifedipine. In high-Ba2+ solution, from negative holding potentials (-70 to -80 mV), nifedipine augmented peak current at test potentials negative to -10 mV and shifted the current threshold in a negative direction; current at potentials positive of -10 mV was reduced. Both effects were concentration dependent. The stimulatory effect of nifedipine was abolished and its inhibitory effect was enhanced when less-negative holding potentials were employed. Bay K 8644 (0.1 or 1 microM) approximately doubled current amplitude in normal-Ca2+ solution at a holding potential of -60 or -70 mV, and shifted maximal peak and threshold current in a negative direction. 5. Holding at more-negative potentials made available a component of current which activated and inactivated at more-negative potentials especially in high-Ba2+ solution.(ABSTRACT TRUNCATED AT 400 WORDS)

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