On the role of endogenous G‐protein βγ subunits in N‐type Ca 2+ current inhibition by neurotransmitters in rat sympathetic neurones

1 Using whole‐cell and perforated‐patch recordings, we have examined the part played by endogenous G‐protein βγ subunits in neurotransmitter‐mediated inhibition of N‐type Ca 2+ channel current ( I Ca ) in dissociated rat superior cervical sympathetic neurones. 2 Expression of the C‐terminus domain of β‐adrenergic receptor kinase 1 (βARK1), which contains the consensus motif (QXXER) for binding Gβγ, reduced the fast (pertussis toxin (PTX)‐sensitive) and voltage‐dependent inhibition of I Ca by noradrenaline and somatostatin, but not the slow (PTX‐insensitive) and voltage‐independent inhibition induced by angiotensin II. βARK1 peptide reduced GTP‐γ‐S‐induced voltage‐dependent and PTX‐sensitive inhibition of I Ca but not GTP‐γ‐S‐mediated voltage‐independent inhibition. 3 Overexpression of Gβ 1 γ 2 , which mimicked the voltage‐dependent inhibition by reducing I Ca density and enhancing basal facilitation, occluded the voltage‐dependent noradrenaline‐ and somatostatin‐mediated inhibitions but not the inhibition mediated by angiotensin II. 4 Co‐expression of the C‐terminus of βARK1 with β 1 and γ 2 subunits prevented the effects of Gβγ dimers on basal Ca 2+ channel behaviour in a manner consistent with the sequestering of Gβγ. 5 The expression of the C‐terminus of βARK1 slowed down reinhibition kinetics of I Ca following conditioning depolarizations and induced long‐lasting facilitation by cumulatively sequestering βγ subunits. 6 Our findings identify endogenous Gβγ as the mediator of the voltage‐dependent, PTX‐sensitive inhibition of I Ca induced by both noradrenaline and somatostatin but not the voltage‐independent, PTX‐insensitive inhibition by angiotensin II. They also support the view that voltage‐dependent inhibition results from a direct Gβγ‐Ca 2+ channel interaction.