Chronotropic action of Na+, K+, Cl- cotransport inhibition in the isolated rat heart.

The chronotropic actions of Na+, K+, Cl- cotransport were investigated by studying the effects of the loop diuretics bumetanide and furosemide, specific inhibitors of the cotransporter, on an isolated rat sino-atrial node preparation. Application of bumetanide decreased the cycle length from 0.334 s (+/- 0.087 S.D.) to 0.279 s (+/- 0.083, n = 16, P = 6.5 x 10(-6)) in Hepes-buffered physiological salt solution (PSS). Similar decreases were recorded in bicarbonate-buffered PSS. Chloride channel blockers indicate that the tachycardia evoked by loop diuretics is not due their blocking of chloride channels. Thus, 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) had a negative chronotropic action and 2-[(2-cyclopentyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1H-inden -5-yl) oxy] acetic acid (IAA-94) produced no change in cycle length. Pharmacological manoeuvres indicate that the positive chronotropic action of loop diuretics is associated with catecholamine release. The positive chronotropic action of bumetanide was inhibited by the beta-adrenoceptor antagonists, propranolol and atenolol, but was unaffected by atropine.