Evidence for Na+/Ca2+ exchange in the rectal gland of Squalus acanthias

Previously we have shown that stimulation of in vitro perfused rectal gland tubules (RGT) of the dogfish Squalus acanthias by adenosine 3',5'-cyclic m onophosphate (cAMP), (as a cocktail comprising 0.1 mmol/l dibutyryl-cAMP, 1 0 mu mol/l forskolin and 0.1 mmol/l adenosine, hereafter termed STIM) leads to an increase in cytosolic Ca2+ ([Ca2+](i)) and that this assists Cl- sec retion by enhancing basolateral Kf conductance. In the present study we exa mined the mechanism of the cAMP-induced increase in [Ca2+](i). [Ca2+](i) wa s measured using the fura-2 technique in isolated in vitro perfused RGT. As before, STIM enhanced [Ca2+](i). This elevation of [Ca2+](i) was prevented completely when STIM was added in the presence of the Na(+)2Cl(-)K(+) cotr ansport inhibitor furosemide (0.5 mmol/l). This suggests that the increase in [Ca2+](i) induced by STIM is caused by a concomitant increase in cytosol ic Na+ ([Na+](i)) and not by the activation of second messenger cascades. F urosemide prevents this increase in [Na+](i) and hence the elevation of [Ca 2+](i). Moreover, the plateau phase of the [Ca2+](i) transient produced by carbachol (CCH, 0.1 mmol/l) was augmented strongly when bath Na+ was reduce d to 5 mmol/l. These data suggest that the level of [Ca2+](i) is determined by Na+-dependent Ca2+ export, most likely via a Na+/Ca2+ exchanger. The;in crease in [Na+](i) accompanying stimulation of Cl- secretion reduces the ra te of Ca2+ export leading to an elevation of [Ca2+](i), as does a reduction in bath Na+ which augments the [Ca2+](i) plateau produced by CCH.