Background: Tubuloglomerular feedback (TGF) is a process whereby the resistance of the afferent arterioles delivering blood to the glomeruli is regulated by the NaCl concentration of the forming urine in the lumen of the macula densa. Intraglomerular mesangial cells are located between capillaries within the glomerulus, while extraglomerular mesangial cells are located between the macula densa and the afferent arteriole. They are electrically and chemically coupled via gap junctions. The purpose of this study was to investigate the role of mesangial cells and gap junctions in TGF using the isolated, perfused juxtaglomerular apparatus.
Method: Juxtaglomerular apparatuses were dissected from male New Zealand white rabbits and perfused in vitro. The NaCl concentration at the macula densa was changed from 17/2 to 65/50 Na/Cl to initiate a TGF response. Afferent arterioles were perfused at 60 mm Hg throughout the experiment. Changes in luminal diameter caused by increasing the NaCl concentration at the macula densa were taken as the TGF response. TGF was measured before and after disrupting the gap junctions or damaging the mesangial cells in paired experiments.
Results: During the control period, TGF decreased afferent arteriole diameter by 2.9 +/- 0.2 microm. After mesangial cells were damaged by perfusing Thy 1-1 antibody and complement into the afferent arteriole, the TGF response was completely eliminated. Separate experiments showed no statistically significant change in TGF response with time, or when antibody and complement were perfused into the macula densa lumen. The presence of Thy 1-1 antibody and complement in the afferent arteriole perfusate did not alter the ability of norepinephrine to constrict or acetylcholine to dilate the afferent arteriole. To investigate the role of gap junctions in TGF, we used heptanol to disrupt them. During the control period, TGF decreased afferent arteriole diameter by 2.9 +/- 0.4 microm. After perfusing heptanol into the lumen of the afferent arteriole, the TGF response was completely eliminated. When heptanol was added to the bath, it had no significant effect on TGF response.
Discussion: The data show that after mesangial cells were selectively damaged, the constriction of the afferent arteriole induced by increasing the NaCl concentration at the macula densa was eliminated. However, such treatment had no effect when Thy 1-1 was perfused into the macula densa lumen, and did not alter the response of the afferent arteriole to norepinephrine or acetylcholine. Disruption of the gap junctions also eliminated the TGF response. These data indicate that the mesangial cells play a key role in mediating the TGF response, and that gap junctions among mesangial cells and between mesangial cells and vascular smooth muscle cells communicate the TGF signal to the afferent arteriole.