Distribution of sympathetic neuroeffector junctions in the juxtaglomerular region of the rabbit kidney

Two structurally distinct types of sympathetic axon (Type I and Type II) have recently been identified in the renal cortex of the rat and the rabbit. This study describes the distribution and density of the neuroeffector junctions made by these two types of axon on the different tissues from the juxtaglomerular region of the rabbit renal cortex. Immunohistochemical studies showed that tyrosine hydroxylase-positive axons were located only in regions adjacent to the arteries and arterioles in the renal cortex. Ultrastructural studies of the juxtaglomerular region indicated that both types of axon formed junctions on vascular smooth muscle cells, epithelial cells of proximal tubules and renin-secreting granular epithelioid cells. The density of neuromuscular junctions (18 × 10 3/mm 2 of vessel surface) was more than twice as high on the afferent arteriole as on the efferent arteriole or proximal tubules immediately adjacent to the glomerular arterioles (both about 6 × 10 3/mm 2). The junction density on granular epithelioid cells was much lower (about 2 × 10 3/mm 2) and were rarely observed on the distal tubule. Afferent arterioles preferentially received junctions from Type I axons at a relatively high density (14.2 × 10 3/mm 2) whereas junctions formed by Type II axons were less selectively distributed and occurred at lower densities on all other tissues (range, 1–6.3 × 10 3/mm 2). Presynaptic membrane specialisations were identified only at junctions on arterioles and granular epithelioid cells and occurred more frequently at Type I than at Type II junctions. The data suggest that the predominant effect of the sympathetic innervation in the juxtaglomerular region of the renal cortex is on the afferent arteriole and that the two axon types within the kidney may have different functions.