Evidence for a modulating effect of NA⁺/H⁺ exchange on the metabolic response of rat brown adipose tissue

Abstract

Membrane potential and intracellular pH (pHi) were simultaneously monitored in rat perifused brown adipose tissue fragments by means of double-barrelled microelectrodes. In parallel experiments, the respiratory rate was measured. The cytosolic pH of unstimulated brown adipocytes was about 0.5 units higher than the value expected for a passive transmembrane distribution of H ions. Isoproterenol (5·10⁻¹⁰ M) had no effect on pHi and membrane potential while it induced a 3.1±0.5-fold increase of the respiratory rate. Clonidine (10⁻⁷ M) alone was followed by a cytosolic alkalinization of 0.14±0.05 pH units with no concomitant increase in the respiratory rate. A mirror image of the intracellular alkalinization induced by clonidine, i.e. an acidification of 0.09±0.03, was noted by monitoring the extracellular pH. Addition of clonidine in the presence of isoproterenol induced an alkalinization of 0.17±0.03 pH units and a 7.7±1.0-fold increase of the respiratory rate. Thus the alkalinizing effect of clonidine developed despite a massive increase in CO₂ production. Pretreatment of the preparation with amiloride (10⁻³ M), an inhibitor of Na⁺/H⁺ exchange, completely prevented the alkalinization and markedly reduced the potentiating effect of clonidine on the isoproterenol-induced respiratory rate. The results obtained are compatible with the hypothesis of a modulating effect of Na⁺/H⁺ exchange on the brown adipocyte metabolic response to catecholamine stimulation.