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

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.