Ultrastructural changes in the sheep renal juxtaglomerular apparatus in response to sodium depletion or loading.

The ultrastructural changes in various components of the renal juxtaglomerular apparatus have been examined in sheep subjected to sodium depletion or loading. In normal sheep, juxtaglomerular arteriolar myoepithelioid cell granulation was relatively sparse whereas peripolar cell granulation was prominent. Sheep subjected to rapid sodium depletion in response to parotid cannula drainage showed features indicating increased activity of the renin-angiotensin system with a rise in plasma renin concentration, an increase in the juxtaglomerular index and morphological evidence of increased renin granule production in arteriolar myoepithelioid cells. In contrast, in sheep subjected to dietary sodium loading, there was evidence of decreased activity of the renin-angiotensin system with a fall in plasma renin concentration and relatively poor myoepithelioid cell granulation. In sodium depleted sheep, juxtaglomerular peripolar cells showed increased synthetic activity and marked heterogeneity of granule density; in sodium loaded sheep, peripolar cells showed no consistent changes. A major new finding was the detection of exocytotic release of granule material from peripolar cells into the urinary space during sodium depletion, confirming the secretory nature of such cells and supporting the concept that they may play a role in sodium homeostasis.