Elevation of cytosolic calcium by imidazolines in mouse islets of Langerhans: implications for stimulus-response coupling of insulin release.
1. Microfluorimetry techniques with fura-2 were used to characterize the effects of efaroxan (200 microM), phenotolamine (200-500 microM) and idazoxan (200-500 microM) on the intracellular free Ca2+ concentration ([Ca2+]i) in mouse isolated islets of Langerhans. 2. The imidazoline receptor agonists efaroxan and phentolamine consistently elevated cytosolic Ca2+ by mechanisms that were dependent upon Ca2+ influx across the plasma membrane; there was no rise in [Ca2+]i when Ca2+ was removed from outside of the islets and diazoxide (100-250 microM) attenuated the responses. 3. Modulation of cytosolic [Ca2+]i by efaroxan and phentolamine was augmented by glucose (5-10 mM) which both potentiated the magnitude of the response and reduced the onset time of imidazoline-induced rises in [Ca2+]i. 4. Efaroxan- and phentolamine-evoked increases in [Ca2+]i were unaffected by overnight pretreatment of islets with the imidazolines. Idazoxan failed to increase [Ca2+]i under any experimental condition tested. 5. The putative endogenous ligand of imidazoline receptors, agmatine (1 microM-1 mM), blocked KATP channels in isolated patches of beta-cell membrane, but effects upon [Ca2+]i could not be further investigated since agmatine disrupts fura-2 fluorescence. 6. In conclusion, the present study shows that imidazolines will evoke rises in [Ca2+]i in intact islets, and this provides an explanation to account for the previously described effects of imidazolines on KATP channels, the cell membrane potential and insulin secretion in pancreatic beta-cells.
Shepherd
R M
RM
Department of Biomedical Science, University of Sheffield, Western Bank.
Hashmi
M N
MN
Kane
C
C
Squires
P E
PE
Dunne
M J
MJ
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000376:Agmatine / Q000378:metabolism; D000818:Animals; D001572:Benzofurans / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D015536:Down-Regulation; D005947:Glucose / Q000378:metabolism; D019329:Idazoxan / Q000494:pharmacology*; D007093:Imidazoles / Q000494:pharmacology*; D054755:Imidazoline Receptors; D007328:Insulin / Q000378:metabolism*; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D010646:Phentolamine / Q000494:pharmacology*; D011955:Receptors, Drug / Q000187:drug effects / Q000378:metabolism; D015398:Signal Transduction
D000317:Adrenergic alpha-Antagonists; D001572:Benzofurans; D007093:Imidazoles; D054755:Imidazoline Receptors; D007328:Insulin; D011955:Receptors, Drug; D000376:Agmatine; C062597:efaroxan; D005947:Glucose; D002118:Calcium; D019329:Idazoxan; D010646:Phentolamine