OSMOTICALLY INDUCED CONDUCTANCE AND CAPACITANCE CHANGES IN IN-VITRO PERFUSED RECTAL GLAND TUBULES OF SQUALUS-ACANTHIAS

The rectal gland of Squalus acanthias is critically involved in the ho meostasis of NaCl and water metabolism and hence in overall osmoregula tion. In the present study, we have examined the acute responses of re ctal gland slices and in vitro perfused rectal gland tubule (RGT) cell s to the exposure to dilute and hypertonic peritubule solutions. Five series were performed. (i) With changes in osmolality, Western blots t o monitor tyrosine, threonine and serine phosphorylation in rectal gla nd slices did not reveal clear-cut changes in phosphorylation patterns . All other series were performed in in vitro perfused RGT. (ii) Relat ive cell volume was estimated by fura-2 fluorescence using the emissio n at the isosbestic excitation wavelength of 360 nm. Hypotonic solutio n (-100 mmol/l NaCl) reduced fura-2 fluorescence by 16% and hypertonic solution (+100 mmol/l NaCl) had the opposite effect (+12%), (iii) Tra nsepithelial resistance was increased markedly by hypotonic solution, probably by cell swelling, and the opposite was seen with hypertonic s olutions. (iv) Whole-cell patch clamp experiments indicated that hypot onic solution hyperpolarized the cells, and increased membrane conduct ance and membrane capacitance. The latter two changes correlated signi ficantly with each other. Hypertonic solution had the opposite effect. (v) Measurements of the fura-2 fluorescence ratio (340/380 nm) reveal ed that hypotonic solution (-NaCl) increased cytosolic Ca2+ activtiy ( [Ca2+](i)). Hypertonic solution had no detectable effect on [Ca2+](i). These data indicate that RGT cells are swollen by removal of NaCl fro m the bath solution. This causes an increase in [Ca2+](i) and a predom inant increase in K+ conductance and hyperpolarization, Urea apparentl y permeates these cells quite well and its addition (+U) or its remova l (-U) had only moderate osmotic effects. The removal of urea and repl acement by mannitol produced effects similar to those seen with hypert onic NaCl solution.