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Abstract

Segments of rectal gland tubules (RGT) the spiny dogfish (Squalus acanthias) were perfused in vitro to study the cellular mechanism by which NaCl secretion is stimulated. Transepithelial PD (PD_te), transepithelial resistance (R_te), the PD across the basolateral membrane (PD_bl), the fractional resistance of the lumen membrane (FR1), and the cellular activities for Cl⁻, Na⁺, and K⁺ (a cellx ) were measured. In series 1 the effects of stimulation (S) (dbcAMP 10⁻⁴, adenosine 10⁻⁴, and forskolin 10⁻⁶ mol · l⁻¹) on these parameters were recorded and compared to nonstimulated state (NS). PD_te increased from −1.9±0.2 mV to −11.0±0.9 mV (n=51). PDbI depolarized from −86±1 to −74±1.4 mV (n=52). R_te fell from 29±2.8 to 21±2 ωcm2 (n=23), and FR₁ fell from 0.96±0.005 to 0.79±0.04 (n=9).a cellK⁺ was constant (123±13 versus 128±17 mmol · 1⁻¹) (n=6), buta cellCl⁻ -fell significantly from 48±4 to 41±3 mmol · l⁻¹ (n=7).a cellNa⁺ increased from 11±2.1 to 29.5±6.6 mmol · l⁻¹ (n=4). In series 2 the conductivity properties were examined by rapid K⁺, and Cl⁻ concentration steps on the basolateral and luminal cell side respectively in NS and S states. In NS-segments reduction of bath K⁺ led to a hyperpolarization of PDbI with a mean slope of 28±1.3 mV/decade (n=9) (as compared to 19 mV/decade for S-state). Reduction of lumen Cl⁻ led to very little depolarization of the lumen membrane PD in NS-state: 6.5±2.3 mV/decade (n=4) (as compared to 13 mV/decade for S-state). In series 3 the effects of furosemide (7 · 10⁻⁵ mol l⁻¹, bath) were examined in NS and S tubules. In NS RGT segments furosemide had no effect on PD_bI or PD_te;a cellCl⁻ fell slowly after furosemide with an initial rate of 0.33 mmol · l⁻1 s⁻¹, as compared to 1.5 mmol · l⁻¹ · s⁻¹ for S-state. The increase ina cellCl⁻ after removal of furosemide from NS to S-states was examined in the presence of furosemide. Despite the presence of furosemide stimulation was accompanied by a fall in R_te, FR₁, anda cellCl⁻ . From these data we conclude that (a) stimulation by cyclic AMP increases the Cl⁻-conductance of the apical cell membrane at least by a factor of 10, that (b) in the NS-state the Na⁺2Cl⁻K⁺ carrier can be triggered to work at rates similar to the S state by lowering a cellCl⁻ , and that (c) the increase in apical Cl⁻-conductance is the primary event in cyclic AMP mediated stimulation of NaCl secretion.