Effect of luminal Cl- and Ca2+ on tubuloglomerular feedback mechanism

We dissected and perfused in vitro rabbit cortical thick ascending limbs (CTAL) and distal convoluted tubule with the glomerulus attached intact. Basolateral membrane voltage (Vbl) and intracellular Ca2+ concentration ([Ca2+]i) in macula densa (MD) cells were measured with microelectrodes and microscope photometry, respectively. Our data suggest that the apical membrane of MD cell has a Ca2+ entry pathway, and that the basolateral membrane of MD cell has a Cl- conductance, which may be regulated by the [Ca2+]i in MD cell. We next evaluated by measuring the afferent arteriolar diameter with video camera monitoring whether the change in Ca2+ concentration of macula densa tubular lumen indeed affect tubuloglomerular feedback. The data suggest that the changes in luminal Ca2+ as well as Cl- concentration at macula densa independently alter the afferent arteriolar diameter, thus may affect the single nephron glomerular filtration rate.