Nucleotide-independent modulation of Ca(2+)-dependent K+ channel current by a mu-type opioid receptor.
Physiological responses to opiates and opioid peptides are transduced via receptors coupled to G proteins. The effectors for these G proteins are often ion channels or second messenger systems that modulate channel activity. In cultured bovine adrenal medullary chromaffin cells (BAMCCs), the activity of a calcium-dependent, voltage-sensitive, potassium (BK) channel is robustly potentiated by a mu-type opioid receptor, an effect consistent with the inhibitory role of opioids versus neural excitability. Patch-clamp electrophysiology was used to investigate coupling between the mu receptor and BK channel, leading to rather surprising results. Potentiation of BK channel activity by the mu-selective agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (10 nM) was unaffected by all attempts to disrupt or alter G protein function, including incubation of cells with pertussis toxin (PTX) and inclusion of guanosine 5'-O-(2-thio)diphosphate (GDP beta S) or guanosine 5'-O-(3-thio)triphosphate (GTP gamma S) in intracellular recording solutions. However, dopamine D2 receptor potentiation of BK current in these same cells was affected by PTX, GDP beta S, and GTP gamma S in predictable fashion. Thus, PTX and GDP beta S inhibited dopamine potentiation of BK current, and GTP gamma S prolonged reversal of dopamine action. These results suggest that the BAMCC BK channel is not coupled to the mu receptor via a GTP-dependent mechanism, whereas in the same cells the dopamine D2 receptor modulates BK channel activity in a conventional GTP-dependent manner. In addition, replacement of both ATP and GTP with nonhydrolyzable analogs also failed to affect either potentiation or recovery of BK channel activity in response to [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin. These results indicate that in BAMCCs the mu-opioid receptor modulates BK channel activity independently of either G proteins or phosphorylation-dependent processes.
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907.
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D000255:Adenosine Triphosphate / Q000502:physiology; D000313:Adrenal Medulla / Q000166:cytology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002417:Cattle; D002478:Cells, Cultured; D019204:GTP-Binding Proteins / Q000502:physiology; D006160:Guanosine Triphosphate / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D010749:Phosphoprotein Phosphatases / Q000502:physiology; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D011494:Protein Kinases / Q000502:physiology; D017450:Receptors, Opioid, mu / Q000187:drug effects / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology; D013237:Stereoisomerism
D015221:Potassium Channels; D017450:Receptors, Opioid, mu; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D011494:Protein Kinases; D010749:Phosphoprotein Phosphatases; D019204:GTP-Binding Proteins; D002118:Calcium