Regulation of the Activity of 27 pS Nonselective Cation Channels in Excised Membrane Patches from Rat Brown-Fat Cells

The regulation of the activity of the approximately 30 pS nonselective cation channel (NSC channel) was studied by the patch-clamp technique in inside-out patches obtained from rat brown-fat cells. NSC channel activity was induced by excision; reduced redox state induced by dithiothreitol accelerated the kinetics in the excised state. The NSC channels were inhibited by the fenamates flufenamic acid and mefenamic acid but not by NS-1619 or SKF-96365. The channels were inhibited by purine nucleotides but not by polyamines. No evidence for protein kinase C, CaM kinase or protein kinase A activation of the NSC channel was obtained. NSC-channel activity was stimulated in a concentration-dependent manner by Ca2+ but the EC50 was very high (0.81 mM), in comparison to expected cytosolic Ca2+ levels. In the presence of ATP, even higher Ca2+ levels were necessary for comparable NSC-channel activation. The increase in Po was not associated with an increase in open-time constants. We conclude that although high Ca2+ levels can experimentally activate the NSC channel, a further mediatory step must probably be postulated in order to link alpha1-adrenergic stimulation to NSC-channel activation.