AJP - Cell Physiology, Vol 261, Issue 1 C177-C184, Copyright © 1991 by American Physiological Society
Agonist-induced frequency modulation of Ca2+ oscillations in salt gland secretory cells
K. M. Crawford, E. L. Stuenkel and S. A. Ernst
Department of Anatomy and Cell Biology, University of Michigan, Ann Arbor 48109.
Oscillations in intracellular calcium concentration ([Ca2+]i) induced by
the acetylcholine analogue carbachol (CCh) were characterized by
microspectrofluorimetry of fura-2 in single secretory cells from the avian
salt gland. The frequency of oscillations increased in graded fashion with
[CCh] between 25 nM (2.7 +/- 0.6 min-1) and 250 nM (11.8 +/- 1.4 min-1),
whereas the amplitude of the spikes was independent of [CCh]. An
interperiod return to prestimulatory [Ca2+]i was generally seen only at
very low (25 nM) CCh. Between 50 and 250 nM CCh, oscillations were
associated with sustained elevated [Ca2+]i levels. The amplitude of the
oscillatory spikes was found not to exceed that of initial spikes arising
from prestimulatory [Ca2+]i, despite the dose-dependent [effective
concentration at 50% (EC50) = 200 nM CCh] sustained rise in [Ca2+]i. At 1
microM CCh, oscillations gave way to a maximal sustained increase in
[Ca2+]i. Reduction of [Ca2+]o to 1.5 microM during an oscillatory train or
blockage of Ca2+ influx with Ni+ resulted in a reduction in sustained Ca2+i
levels and in frequency, but not amplitude, of oscillations. A relationship
between the sustained partial rise in [Ca2+]i derived from Ca2+ influx and
the oscillatory frequency at a given [CCh] was further indicated by the
lower frequency (P less than 0.01) of the early spikes in a train when
interspike [Ca2+]i initially returned to near-basal levels. In some cells,
oscillations were slow enough (less than 2 min-1) to resolve an interperiod
of elevated baseline [Ca2+]i, showing that the latter can occur independent
of the repetitive Ca2+ spikes. (ABSTRACT TRUNCATED AT 250 WORDS)
