Inhibition of calcium currents by noradrenaline, somatostatin and opioids in guinea-pig submucosal neurones.
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.
Surprenant
A
A
Vollum Institute, Oregon Health Sciences University, Portland 97201.
Shen
K Z
KZ
North
R A
RA
Tatsumi
H
H
D000200:Action Potentials / Q000187:drug effects; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects; D002432:Cecum / Q000294:innervation; D003864:Depression, Chemical; D004744:Enkephalin, Methionine / Q000494:pharmacology*; D019204:GTP-Binding Proteins / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006168:Guinea Pigs; D007082:Ileum / Q000294:innervation; D066298:In Vitro Techniques; D009474:Neurons / Q000502:physiology; D009638:Norepinephrine / Q000494:pharmacology*; D011941:Receptors, Adrenergic / Q000502:physiology; D011957:Receptors, Opioid / Q000502:physiology; D012684:Sensory Thresholds / Q000502:physiology; D013004:Somatostatin / Q000494:pharmacology*; D013368:Submucous Plexus / Q000502:physiology*
D000316:Adrenergic alpha-Agonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D011941:Receptors, Adrenergic; D011957:Receptors, Opioid; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D013004:Somatostatin; D004744:Enkephalin, Methionine; D019204:GTP-Binding Proteins; D002118:Calcium; D009638:Norepinephrine