ELEMENTAL COMPOSITION OF NA PUMP INHIBITED RABBIT AORTA VSM CELLS BY ELECTRON-PROBE X-RAY-MICROANALYSIS

The Na pump in vascular smooth muscle (VSM) is likely to influence not only intracellular Na content but also the content and distribution o f other cations and anions measured by electron probe X-ray microanaly sis (EPXMA). The hypothesis we tested was that EPXMA of pump-inhibited VSM would yield a characteristic cellular elemental profile, providin g insight into the contribution of the Na pump to the intracellular mi lieu and an approach to identifying when VSM operates under the constr aints of pump inhibition. We assessed the contractile state and elemen tal EPXMA profile of rabbit aorta that was either quiescent or contrac ted by serotonin (10(-6) M) or ouabain (10(-6) M). VSM cytoplasm showe d the anticipated low Na (28 +/- 2 mM) and high K (182 +/- 5 mM) conte nt. With ouabain, Na rose and K fell to reverse the Na-to-K ratio (0.1 5 +/- 0.01 vs. 6.6 +/- 0.3; P < 0.01). With serotonin, the ratio rose slightly (0.28 +/- 0.2; P < 0.05). Nuclei and mitochondria showed a si milar pattern. Cl showed a small increase (56 +/- 2 to 102 +/- 4 mM) w ith ouabain, a shift that could not be accounted for on the basis of c harge redistribution to maintain neutrality because the change in Na a nd K were essentially offsetting. EPXMA measures total and not ionized Ca. If changes in cytoplasmic Ca occurred, they were too small to be measured by the imaging methods employed. The sustained, myogenic cont ractile response of VSM to Na pump inhibition shows a characteristic e lemental profile that could prove useful in its identification. Direct measurement of membrane potential during the myogenic response to Na pump inhibition should have a high priority.