Two types of Ca2+ currents with different sensitivities to organic Ca2+ channel antagonists in guinea pig pancreatic alpha 2 cells.
The possibility that guinea pig pancreatic alpha 2 cells are equipped with more than one type of Ca2+ channel was explored using the patch-electrode voltage-clamp technique. At a holding potential of -100 mV, a slowly developing (tau m approximately 5 ms at -40 mV assuming m2 kinetics) Ca2+ current appeared. This conductance first became detectable at potentials of about -60 mV and reached a maximum amplitude of 50-100 pA between -30 and -20 mV. During long depolarizations, it inactivated completely (tau h approximately 100 ms at -40 mV). Half-maximal steady state inactivation was observed at about -60 mV. A second, more rapidly developing (tau m approximately 2 ms at 0 mV) Ca2+ current was observed during pulses to -40 mV and above. It had a peak amplitude of 150-200 pA between 0 and 10 mV, was less dependent on the holding potential, and inactivated very little, even during long pulses. Both conductances were blocked by Co2+ but were unaffected by tetrodotoxin. The rapidly developing current differed from the slowly developing one in being sensitive to the antagonists D-600 and nifedipine, conducting Ba2+ better than Ca2+, increasing upon exposure to forskolin, and showing time-dependent decay (rundown). These findings indicate that the alpha 2 cells are equipped with two kinds of Ca2+ channels.
D000818:Animals; D001464:Barium / Q000378:metabolism; D002118:Calcium / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D003035:Cobalt / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D004594:Electrophysiology; D005711:Gallopamil / Q000494:pharmacology; D006168:Guinea Pigs; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007515:Islets of Langerhans / Q000166:cytology / Q000502:physiology*; D008564:Membrane Potentials; D009543:Nifedipine / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D002121:Calcium Channel Blockers; D007473:Ion Channels; D005576:Colforsin; D001464:Barium; D005711:Gallopamil; D003035:Cobalt; D013779:Tetrodotoxin; D009543:Nifedipine; D002118:Calcium