Abstract:
: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.
journal_name
Mol Pharmacoljournal_title
Molecular pharmacologyauthors
Eddlestone GT,Komatsu M,Shen L,Sharp GWsubject
Has Abstractpub_date
1995-04-01 00:00:00pages
787-97issue
4eissn
0026-895Xissn
1521-0111journal_volume
47pub_type
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