CALCIUM TRANSIENTS EVOKED BY ACTION-POTENTIALS IN THE SOMATA OF CHICKCILIARY NEURONS

The effect of action potentials on the calcium concentration in the so mata of chick ciliary neurons ([Ca2+](s)) was determined by loading th ese with the calcium indicator calcium green-1. Following trains of 1- 10 impulses (30 Hz) to the postganglionic nerve, the [Ca2+](s) increas ed rapidly and then declined along a single exponential with a time co nstant of 0.70 +/- 0.04 s (fast phase). After trains of 20 or 50 impul ses, the elevated [Ca2+](s) declined as the sum of two exponentials, w ith time constants of 0.78 +/- 0.12 s (fast phase) and 4.0 +/- 0.4 s ( moderate phase). After a 600-impulse postganglionic train of impulses, the elevated [Ca2+](s) declined quickly over about 1 s, and then as t he sum of two exponentials: that of the moderate phase and a slower co mponent with a time constant of 109 +/- 16 s (slow phase). Similar tim e courses were observed following stimuli to the preganglionic nerve. Caffeine (3 mM) and ryanodine (20 mu M) both sped the fast phase and s lowed the moderate phase of [Ca2+](s) decline. Carbonyl cyanide m-chlo rophenyl hydrazone (CCCP, 2 mu M) slowed the slow phase, without affec ting the other phases of decline. These results are discussed in relat ion to identifying the mechanisms responsible for these different phas es of Ca2+ removal. (C) 1998 Elsevier Science B.V. All rights reserved .