Mastoparan increases the intracellular free calcium concentration in two insulin-secreting cell lines by inhibition of ATP-sensitive potassium channels.
The mechanisms underlying mastoparan-induced elevation of the intracellular free calcium concentration ([Ca2+]i) were investigated in the insulin-secreting cell lines RINm5F and HIT. In both cell types, micromolar concentrations of mastoparan induced a prompt increase of [Ca2+]i, measured as an increase in fura-2 fluorescence. This response was dependent on extracellular calcium entry and was suppressed by organic calcium channel blockers; the increase of [Ca2+]i caused by high glucose concentrations or tolbutamide was not enhanced by mastoparan. These data indicate the involvement of voltage-dependent calcium channels and suggest that depolarization, rather than a direct effect on the channels, mediates the response to mastoparan. This proposition was supported by the observation that whole-cell calcium currents measured using the nystatin-permeabilized patch technique were not affected by mastoparan. Mastoparan-induced depolarization was observed using the potentiometric indicator bis-oxonol, and it was shown not to be additive with the depolarization induced by high glucose concentrations or tolbutamide. The mechanism underlying mastoparan-induced depolarization was identified in single-channel patch-clamp experiments, where it was shown that mastoparan caused closure of ATP-sensitive potassium channels [K(ATP) channels] in cell-attached and excised membrane patches. Responsiveness to mastoparan in excised patches demonstrated the membrane-delimited character of K(ATP) channel inhibition. The observation that the response persisted in the absence of exogenous GTP and in the presence of 250 microM GDP or guanosine-5'-O-(2-thio)diphosphate suggested that this effect is not mediated via enhancement of G protein activity. Partial suppression of channel activity by mastoparan did not prevent the action of tolbutamide, which fully suppressed the remaining activity in excised patches. In summary, the increase of [Ca2+]i in the insulin-secreting tumor cell lines RINm5F and HIT in response to mastoparan is mediated via G protein-independent suppression of K(ATP) channel activity, cell depolarization, and activation of voltage-dependent calcium channels.
Eddlestone
G T
GT
Department of Pharmacology, College of Veterinary Medicine, Cornell University, Ithaca, New York 14850, USA.
Komatsu
M
M
Shen
L
L
Sharp
G W
GW
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects; D006160:Guanosine Triphosphate / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D036341:Intercellular Signaling Peptides and Proteins; D008564:Membrane Potentials / Q000187:drug effects; D010455:Peptides; D026902:Potassium Channel Blockers; D051381:Rats; D014407:Tumor Cells, Cultured; D014862:Wasp Venoms / Q000494:pharmacology*
D015220:Calcium Channels; D007328:Insulin; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D026902:Potassium Channel Blockers; D014862:Wasp Venoms; C037500:mastoparan; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D002118:Calcium