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

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Abstract
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.