EVIDENCE FOR THE CALCIUM-DEPENDENT POTENTIATION OF M-CURRENT OBTAINEDBY THE RATIOMETRIC MEASUREMENT OF THE FURA-2 FLUORESCENCE IN BULLFROGSYMPATHETIC NEURONS

Intracellular Ca2+ concentration ([Ca](i)) was measured following the activation of an inward Ca2+ current and subsequent potentiation of an M-type K+ current (I-M) in bullfrog sympathetic neurons. Fura-e was u sed as an indicator for [Ca](i). The fluorescence ratio at 340 and 380 nm (F-340/F-380) was elevated from 0.36 to 1.22 when I-M was potentia ted by 68% following the Ca2+ current. Based on the in vivo calibratio n curve obtained from cells permeabilized with digitonin (20 mu M), th e F-340/F-380 value of 1.22 was equivalent to a [Ca](i) of 0.97 mu M. We therefore propose that a rise in [Ca](i) into the micromolar range can lead to the potentiation of I-M in amphibian autonomic neurons. (C ) 1997 Elsevier Science Ireland Ltd.