Calcium and magnesium transport by in situ mitochondria: electron probe analysis of vascular smooth muscle

The extent, time course, and reversibility of mitochondrial Ca2+ uptake secondary to cellular Ca2+ influx stimulated by massive Na+ efflux were evaluated by electron probe microanalysis of rabbit portal vein smooth muscle. Strips of portal vein were Na+ loaded for 3 hours at 37 degrees C in a K+-free 1 mM ouabain solution, after which rapid Na+ efflux was induced by washing with a Na+-free K+-Li+ solution (1 mM ouabain). Li+ washing Na+-loaded portal vein produced a large transient contraction accompanied by an increase (over 100-fold) in mitochondrial Ca2+ and also significant (p less than 0.05) increases in phosphorus and Mg2+. The Ca2+ loading of the mitochondria was reversed during prolonged Li+ wash, and by 2 hours, mitochondrial Ca2+, Mg2+, and phosphorus had returned to control levels. The maximal contractile response to stimulation remained normal, demonstrating that pathologic Ca2+ loading of mitochondria is reversible in situ and compatible with normal maximal force developed by the smooth muscle. Mitochondrial Ca2+ and phosphorus uptake were reduced but still significant when the Li+ wash contained 0.2 mM Ca2+ or when ouabain was omitted. The fact that mitochondrial Ca2+ loading accompanied submaximal contractions during 0.2 mM Ca2+-Li wash suggests "supranormal" affinity of mitochondria for Ca2+ and may be due, in part, to reverse operation of the mitochondrial Na+-Ca2+ exchanger. Mitochondrial Ca2+, Mg2+, and phosphorus uptake were eliminated when the Li+ wash was performed at 2 degrees C or when the wash contained no Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)