Inhibition of calcium currents by noradrenaline, somatostatin and opioids in guinea-pig submucosal neurones.

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1. Whole-cell recordings were made from submucosal neurones acutely dissociated from guinea-pigs. The actions of noradrenaline, somatostatin and [Met5]enkephalin on currents carried by calcium ions were studied. 2. On depolarization from a holding potential of -70 mV, an inward current activated at -40 mV, reached its peak amplitude at 10 mV and reversed to outward at 72 mV (with external calcium of 5 mM and internal caesium of 160 mM). 3. Cadmium, nickel and cobalt reversibly blocked the calcium current; concentrations causing 50% block were 2.5, 500 and 2000 microM respectively. The calcium current (holding at -70 or -30 mV) was reversibly blocked by omega-conotoxin (100 nM), and unaffected by Bay K 8644 (0.1-10 microM) and nifedipine (1 microM). Cadmium caused an outward shift in holding current at -30 mV, implying that there was a persistent inward calcium current at this potential. 4. Noradrenaline, somatostatin and [Met5]enkephalin decreased the calcium current. The maximal inhibition observed with any one agonist, or with a combination of two agonists, did not exceed 50%; concentrations giving half-maximal inhibition were 5.5 microM for noradrenaline, 4 nM for somatostatin and 1 microM for [Met5]enkephalin. The inhibition was independent of membrane potential. All three agonists also reduced the persistent calcium current at -30 mV. 5. Inhibition of the calcium current by noradrenaline occurred with a latency of not less than 175 ms; cadmium applied by the same method depressed the current within 5-45 ms. 6. Experiments with selective agonists and antagonists indicated that the receptor types involved in calcium current inhibition were alpha 2-adrenoceptors and delta-opioid receptors. Somatostatin acted at a distinct receptor. 7. Calcium currents were also inhibited by intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S). Agonists were ineffective in cells pre-treated with pertussis toxin, but their action was restored when purified GTP-binding proteins (Go or Gi) were included in the intracellular recording solution. 8. It is concluded that noradrenaline, somatostatin and [Met5]enkephalin act at their respective receptors on guinea-pig submucosal neurones to inhibit a voltage-dependent calcium current. Activation of the same receptors also increases a potassium conductance in these cells: in both cases a pertussis-sensitive G protein is involved.

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