Membrane damage and the Ca(2+)-paradox in the perfused rat kidney

Lactate dehydrogenase (LDH) leaks from the perfused rat kidney under the artificial conditions of a Ca(2+)-paradox protocol, namely Ca(2+)-repletion following a 20 minute period of Ca(2+)-depletion. LDH leakage was markedly suppressed by perfusion at 25 degrees C or with 0.1 mM dibucaine or 2 mM lidocaine. Lidocaine inhibited leakage only during Ca(2+)-depletion. Lowering the perfusion rate significantly reduced LDH escape. No LDH loss occurred if the osmotic pressure of the perfusion fluid was raised by 420 mOsm during either Ca(2+)-depletion or Ca(2+)-repletion. Amiloride (2 mM) significantly reduced LDH leakage to 43%. Reduction of the pH of the perfusion fluid to 6.8 significantly inhibited LDH loss, and at pH 6.4 this leakage was almost completely suppressed. LDH loss was equally suppressed at pH 6.4 only during Ca(2+)-depletion, whereas pH 6.4 was markedly less effective when perfused only during Ca(2+)-repletion. Ouabain (5 x 10(-6) M) had only a limited effect in exacerbating LDH leakage. Raising [K+]o significantly protected against LDH leakage, which fell to 36% at 16 mM [K+]. These features correspond with the Ca(2+)-paradox of the perfused rat heart an it is suggested that: (i) a Ca(2+)-paradox can be produced in the rat kidney; (ii) a similar mechanism governs the release of cytosolic proteins in these two preparations; and (iii) the damage mechanism of the plasmalemma is a transmembrane oxidoreductase-diaphorase molecular complex which generates H+ when activated by Ca(2+)-depletion.