Inhibitory effects of chemically-different 'loop' diuretics on chloride transport across the bullfrog cornea

Frog (Rana catesbeiana) corneas were mounted in an Ussing chamber, modified to facilitate dissection and to avoid edge damage to the epithelial tissue during mounting and measurements. When bathed in Conway solution of pH 7.4 the corneas displayed highly stable electrical properties, with a transepithelial potential difference (PD) of 16.6 +/- 1.O mV and a short-circuit current (SCC) of 10.3 +/- 0.8 microA cm-2. The DC resistance was 2.0 +/- 0.15 k omega X cm2 (mean +/- SE, n = 45). An increase in the intracellular cyclic AMP level induced by prostaglandin E2 and by 3-isobutyl-1-methylxanthine (IBMX) resulted in an increase in SCC. Ortho-vanadate, an inhibitor of Na+-K+-ATPase, reduced SCC. The acidic loop diuretics furosemide, bumetanide and the levorotatory form of indacrinone (MK-196) reduced SCC by about 50% at concentrations of 500, 100 and 46 microM, respectively. The (-)form of MK-196 was four times more active than the (+)form. Dimethylation of the SO2NH2 group reduced the activity of bumetanide. The basic diuretics muzolimine (Bay G 2821) and MK-447, were found, similarly, to reduce SCC by about 50% at concentrations of 500 microM. In view of their 'high ceiling' type of saluretic effects in whole animals, these basic agents should therefore be classified as 'loop' diuretics. The effects of these structurally highly different 'loop' diuretics are similar in epithelia which secrete (cornea) and in those which absorb (the renal thick ascending limb of Henle's loop; TALH) chloride ions.(ABSTRACT TRUNCATED AT 250 WORDS)