Morphological changes in the renal macula densa during natriuresis and diuresis.

Morphological changes in the macula densa have been studied during the infusion of diuretic agents into the renal artery of anesthetized dogs. The kidneys were fixed by rapid high pressure perfusion with glutaraldehyde. Large basolateral intercellular spaces were seen between macula densa cells in control kidneys, but the number and extent of these spaces were strikingly reduced during the natriuresis and diuresis induced by the infusion of frusemide, ethacrynic acid or mannitol. Natriuresis and diuresis produced by the intravenous infusion of large volumes of 0.9% NaCl solution also resulted in closure of these spaces. No simple relationship existed between changes in plasma renin activity and closure of the spaces between macula densa cells during these procedures. A distinctive, membrane-bound, vesicle-containing structure was identified between the basolateral processes of the cells of each macula densa; the function of this structure awaits elucidation. We suggest that changes in the size of the basolateral intercellular spaces of the macula densa reflect changes in fluid flux between the distal tubule and the interstitium of the juxtaglomerular apparatus.