M. Salomonsson

Macula densa cells sense luminal NaCl concentration via furosemide sensitive Na+2Cl−K+ cotransport

The macula densa cells of the juxtaglomerular apparatus probably serve as the sensor cells for the signal which leads to the appropriate tubuloglomerular feedback response. The present study reports basolateral membrane voltage (PDbl) measurements in macula densa cells. We isolated and perfused in vitro thick ascending limb segments with the glomerulus, and therefore the macula densa cells, and the early distal tubule still attached. Macula densa cells were impaled with microelectrodes under visual control. PDbl was recorded in order to examine how these cells sense changes in luminal NaCl concentrations. The addition of furosemide, a specific inhibitor of the Na+2Cl−K+ cotransporter in the thick ascending limb, to the lumen of the perfused thick ascending limb hyperpolarized PDbl from −55±5 mV to −79±4 mV (n=7). Reduction of NaCl in the lumen perfusate from 150 mmol/l to 30 mmol/l also hyperpolarized PDbl from −48±3 mV to −66±5 mV (n=4). A Cl− concentration step in the bath from 150 mmol/l to 30 mmol/l resulted in a 24±4 mV (n=4) depolarization of PDbl. This depolarization of PDbl was absent when furosemide was present during the Cl− concentration step. These data suggest that the macula densa cells sense changes in luminal NaCl concentration via coupled uptake of Na+ and Cl−. The transport pathways for NaCl transport in macula densa cells are probably identical to those in the thick ascending limb: the (Na++K+)-ATPase in the basolateral membrane drives Na+ and Cl− uptake via the luminal Na+2Cl−K+ cotransport, Cl− leaves the cell via basolateral Cl− channels and K+ recycles across the apical membrane via K+ channels. Changes in intracellular Cl− activity as a result of altered luminal NaCl uptake, and thus voltage changes of the basolateral membrane are probably the first signal in the tubuloglomerular feedback regulation.

Lack of effect of intraluminal pressure on renin release from isolated afferent arterioles

To evaluate the role of the proposed baroreceptor mechanism in the afferent arteriole in regulating renin release, we modified the isolated perfused tubule technique to perfuse afferent arterioles. Arterioles with attached glomeruli were isolated from rabbit kidneys and perfused using standard methods. To stop the arteriolar flow and allow perfusion pressure, as set by a mercury manometer, to be built up in the lumen of the vessel, the glomerulus was sucked into a constriction pipette. The preparation was continuously superfused with Krebs-Ringer solution in the first series of experiment, and a cell culture medium in the second series of experiment. The superfusate droplets were collected under mineral oil with 10-min collection intervals. The renin content of the samples was assayed by radioimmunoassay of the angiotensin I generated. In the two series of experiments we tested the effects of sequential changes in intraluminal pressure on renin release. In the first series of experiments (n=6) the renin release was 56.3 nGU arteriole−1 min−1 in the first 10 min of sampling. The renin release was then constant for 80 min with an average of 21.6 nGU arteriole−1 min−1. In the last 30 min the renin release was 96.5 nGU arteriole−1 min−1. In the second series of experiments (n=8) the renin release was 26.5 nGU arteriole−1 min−1 throughout the course of the experiment. These results indicate that under these conditions there is no relation between renin release and intraluminal pressure in afferent arterioles.