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

Abstract.

Previously we have shown that stimulation of in vitro perfused rectal gland tubules (RGT) of the dogfish Squalus acanthias by adenosine 3',5'-cyclic monophosphate (cAMP), (as a cocktail comprising 0.1 mmol/l dibutyryl-cAMP, 10 µmol/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^– secretion by enhancing basolateral K⁺ conductance. In the present study we examined the mechanism of the cAMP-induced increase in [Ca²⁺]_i. [Ca²⁺]_i was 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⁺ cotransport inhibitor furosemide (0.5 mmol/l). This suggests that the increase in [Ca²⁺]_i induced by STIM is caused by a concomitant increase in cytosolic Na⁺ ([Na⁺]_i) and not by the activation of second messenger cascades. Furosemide prevents this increase in [Na⁺]_i and hence the elevation of [Ca²⁺]_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 reduced 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 increase in [Na⁺]_i accompanying stimulation of Cl^– secretion reduces the rate 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.