The effects of bicarbonate and foreign anions on chloride transport in smooth muscle of the guinea-pig vas deferens

Abstract

The selectivity of the external site of the Cl transporting mechanism in the guinea-pig vas deferens has been investigated by measurement of 36Cl uptake and efflux and by direct measurement of intracellular pH. Replacing 50% of the Cl in normal Krebs solution inhibited the 15 min uptake of 36Cl in the order NO3 greater than Br greater than SCN greater than F greater than I greater than glucuronate, both in Cl-depleted tissues and tissues pre-incubated in the 50%-Cl solutions (steady-state uptake). After 3 h incubation in these solutions, the total cellular Cl was reduced by the anions in the order Br greater than NO3 greater than I greater than SCN greater than F greater than glucuronate. Br, NO3 and I reduced the cellular Cl to less than 50% of normal, suggesting that they are actively taken up by the cells. The ability of foreign anions to inhibit a 3 min uptake of high specific activity, low concentration Cl (6.5 mM) suggests an apparent affinity series of NO3 greater than Cl = SCN = Br greater than I greater than F at the external site. Addition of NO3, Cl, Br, HCO3, F, SCN or I to a Cl-free, nominally HCO3-free bathing solution accelerated 36Cl efflux. The first four mentioned were powerful stimulants, the other three less potent. However, the exact position of HCO3 in the sequence is uncertain. The rapidity with which CO2 crosses the membrane and forms HCO3 intracellularly may allow competition between HCO3 and Cl at the internal site and so distort the result. The action of F is also questionable since this ion drastically reduces the divalent cation activity and is a metabolic inhibitor. Measurement of intracellular pH provided conclusive evidence that Cl, NO3, Br and I can exchange with HCO3 across the cell membrane. This exchange is as rapid with NO3 as with Cl but slower with Br and considerably slower with I. The results also indicate that SCN ions cross the cell membrane. It is concluded that Cl, HCO3, Br and NO3 are all translocated by the exchange carrier. I and perhaps SCN also interact with the transport mechanism, but the translocation rate is then greatly reduced. The precise order of the affinity of these anions remains uncertain but the following sequence: NO3 greater than Cl = SCN = Br greater than I greater than F is considered to be the most likely.