ADH or theophylline-induced changes in intracellular free and membrane-bound calcium

Abstract

Ca2+ is thought to play a role in the enhancement of water permeability of toad urinary bladder epithelial cells by antidiuretic hormone (ADH) or theophylline. This study examined the effects of ADH and theophylline on intracellular free Ca2+ ([Ca2+]i) and total cellular exchangeable Ca2+ in isolated toad bladder epithelial cells. ADH or theophylline enhanced water permeability maximally by 15-25 min after a 4-min lag. 45Ca2+ efflux, a probe for total cellular exchangeable (plasma membrane plus intracellular) Ca2+, was enhanced by ADH within 2 min and returned to control by 8 min. Chlortetracycline fluorescence, a probe for intracellular Ca2+ only, was not affected, suggesting that ADH released only plasma membrane-bound Ca2+. Theophylline enhanced 45Ca2+ efflux and decreased chlortetracycline fluorescence, suggesting release of Ca2+ from intracellular sources. Both agents decreased [Ca2+]i as assessed by quin-2 fluorescence with a time course similar to the enhancement in water permeability. The results suggest that the changes in membrane-bound Ca2+ and [Ca2+]i induced by ADH and theophylline may play a role in the enhanced permeability to water in response to these agents.

Fluorescent measurements of intracellular free calcium in isolated toad urinary bladder epithelial cells

“…[36] but lower than noncorrected values [9,47]. At higher Quin 2 loading (50-180 nmol/mg protein), an apparent minimum value for Casof 58 _+ 8.4 nM (n = 8) was observed, a value consistent with Ca s measured in Quin 2 loaded toad bladder cells incubated in medium containing 0.1 mM Ca [36].…”

Direct inhibition of epithelial Na+ channels by a pH-dependent interaction with calcium, and by other divalent ions

“…Moreover, since the Ca 2 § inhibition curve is not cooperative, a half-maximal decrease in Na + conductance would require at least a 10-fold increase in cell Ca 2+. A change of that magnitude was not observed experimentally (Crutch & Taylor, 1983;Burch & Halushka, 1984;Chase & Wong, 1984). On the other hand, a similar decrease in conductance can be achieved by increasing the internal pH by 0.2 units (at constant Ca 2+ activity of 0.5/xM), a value which is well within the physiological fluctuations of cytoplasmic pH.…”