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Keywords:

  • α adrenoceptors;
  • opioids;
  • desensitization;
  • cAMP;
  • G-protein

Abstract

Voltage-sensitive calcium currents were recorded from chemically differentiated neuroblastoma × glioma hybrid (NG108-15) cells using the whole-cell clamp technique. Both noradrenaline and [D-Ala2, D-Leu5] enkephalin (DADLE) reversibly depressed the amplitude of the calcium current by up to 30%. The response to noradrenaline occluded that to DADLE suggesting that both agonists depress the same fraction of current. The response to DADLE but not that to noradrenaline desensitized rapidly. Cells responded normally to noradrenaline when desensitized to the opioid. Responses to either agonist were absent in cells pre-incubated with pertussis toxin. In addition the response to noradrenaline became irreversible in cells dialysed internally with a non-hydrolysable analogue of GTP. The response to noradrenaline was not affected by treatment of the cells with either membrane-permeable analogues of cAMP or a combination of forskolin and isobutylmethylxanthine. It is concluded that both noradrenaline and DADLE depress the same fraction of voltage-dependent calcium current in NG108-15 cells; that the responses are mediated by a pertussis-sensitive GTP-binding protein but are not secondary to a reduction in the intracellular concentration of cAMP; and that desensitization of the opioid response occurs at a site linked intimately to the opioid receptor rather than at a common site in the transduction pathway between receptor activation and reduction in the calcium channel current.