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Department of Anatomy & Physiology, University of Dundee, UK.

1. Chloride accumulation into rat saphenous arterial smooth muscle has been examined using chloride-sensitive microelectrodes, to assess the contribution of Na(+)-Cl-cotransport. 2. Bumetanide (10 microM) produced a fall in intracellular chloride ([Cl-]i), and a hyperpolarization of membrane potential (Em). However, [Cl-]i remained above the equilibrium level predicted from the membrane potential, indicating a residual accumulation. 3. Replacement of extracellular sodium with N-methyl-D-glucamine or choline caused a fall in [Cl-]i similar to that observed with bumetanide, but the hyperpolarization of Em was larger. In Na(+)-free media, bumetanide had no effect. [Cl-]i remained significantly above equilibrium. 4. In the presence of bumetanide, chlorothiazide produced a further dose-dependent fall in [Cl-]i, and hyperpolarization of Em. However, although [Cl-]i fell more than with bumetanide alone, it remained significantly above equilibrium. Metolazone was without effect at 100 microM. 5. In the presence of bumetanide, ethacrynic acid and N-ethyl maleimide caused a dose-dependent hyperpolarization of Em and a fall in [Cl-]i to equilibrium. 6. The third inward chloride pump in rat saphenous arterial smooth muscle appears not to be a form of Na(+)-Cl- cotransport. The potency series of thiazide diuretic action (acetazolamide > chlorothiazide > metolazone) differed significantly from that published for Na(+)-Cl- cotransport, and there is no sodium dependence.