Electrophysiological and molecular characterization of the inward rectifier in juxtaglomerular cells from rat kidney

Abstract

Renin, the key element of the renin-angiotensin-aldosterone system, is mainly produced by and stored in the juxtaglomerular cells in the kidney. These cells are situated in the media of the afferent arteriole close to the vessel pole and can transform into smooth muscle cells and vice versa. In this study, the electrophysiological properties and the molecular identity of the K + channels responsible for the resting membrane potential (∼ −60 mV) of the juxtaglomerular cells were examined. In order to increase the number of juxtaglomerular cells, afferent arterioles from NaCl-depleted rats were used, and > 90% of the afferent arterioles were renin positive at the distal end of the arteriole. Whole-cell and cell-attached single-channel patch-clamp experiments showed that juxtaglomerular cells are endowed with a strongly inwardly rectifying K + channel (Kir). The channel was highly sensitive to inhibition by Ba 2+ (inhibition constant 37 µM at 0 mV), but relatively insensitive to Cs + and, with 142 mM K + in the pipette, had a single-channel conductance of 31.5 pS. Immunocytochemical studies showed the presence of Kir2.1 but no signal for Kir2.2 in the media of the afferent arteriole. In PCR analyses using isolated juxtaglomerular cells, the mRNA for Kir2.1 and Kir2.2 was detected. Collectively, the results show that Kir2.1 is the dominant component of the channel. The current carried by these channels plays a decisive role in setting the membrane potential of juxtaglomerular cells.

Cyclic AMP increases cytoplasmic free calcium in renin-secreting cells from rat kidney

“…In addition, the vessels were prepared from NaCl-depleted rats in order to maximise the number of renin-secreting cells (Taugner and Hackenthal 1989). We have indeed found that, the vast majority of cells that we observe under these conditions at the vessel pole are renin-positive (Leichtle et al 2004). In the more proximal parts of the afferent arteriole, however, smooth muscle cells predominate and the responses to stimulation by ISO + IBMX were compared in the two regions (distal: 0-20 μm, proximal: 60-100 μm from the entrance into the glomerulus).…”

“…This is of importance, as the cells in this preparation are strongly coupled by gap junctions (Kurtz and Penner 1989;Russ et al 1999;Leichtle et al 2004). A disadvantage of the preparation is the absence of an intravascular pressure which results in a negative membrane potential (from -60 to -65 mV (Kurtz and Penner 1989;Leichtle et al 2004)) and low values of [Ca 2+ ] i (Laske-Ernst et al 2008). Furthermore, the afferent arteriole is composed of smooth muscle cells and JGC and, at least in principle, one cannot be certain from which cell type the signal arises.…”

“…1). These cells were clearly distinguishable from smooth muscle cells which have a spindle-like appearance and a spiral arrangement around the vessel (Bührle et al 1984;Hackenthal et al 1990); instead, their roundish shape suggested them to be renin-secreting cells (Russ et al 1999;Leichtle et al 2004;Laske-Ernst et al 2008). About 38 pictures resulting in 19 ratios per minute were recorded and stored.…”

“…However, these two cell types mutually transform into each other according to the demands of the body (Cantin et al 1977;Hackenthal et al 1990;Sequeira Lopez et al 2001;Sequeira Lopez et al 2004). Therefore, we have increased the number of JGC by NaCl-depleted diet and limited the region of interest to the most distal part of the arteriole where JGC predominate (Hackenthal et al 1990;Leichtle et al 2004) such that the signal essentially reflects [Ca 2+ ] i in JGC. On the other hand, the data in Fig.…”

“…The experiments were performed in vessels prepared from rats kept on a NaCldepleted diet; under these conditions the number of JGC is greatly increased and the cells contributing to the signal near the vessel pole are mostly JGC (Hackenthal et al 1990;Leichtle et al 2004). Hence, one might speculate that in rat JGC, the stimulating action of cAMP on renin secretion is fine-tuned by a concomitant increase in [Ca 2+ ] i which reduces secretion, resulting in a more balanced stimulation of the RAAS.…”