Mechanism of NaCl secretion in rectal gland tubules of spiny dogfish (Squalus acanthias)

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_x^cell) 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). PD_bI depolarized from –86±1 to –74±1.4 mV (n=52). R_te fell from 29±2.8 to 21±2 cm² (n=23), and FR₁ fell from 0.96±0.005 to 0.79±0.04 (n=9).a_K+^cell was constant (123±13 versus 128±17 mmol · 1^–1) (n=6), buta_Cl–^cell-fell significantly from 48±4 to 41±3 mmol · l^–1 (n=7).a_Na+^cell 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 PD_bI 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_Cl–^cell 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_Cl–^cell 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_Cl–^cell. 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 loweringa_Cl–^cell, and that (c) the increase in apical Cl^–-conductance is the primary event in cyclic AMP mediated stimulation of NaCl secretion.Supported by Deutsche Forschungsgemeinschaft Gr 480/8-1, and by NIH Grant AM 34208