Endothelin synthesis and receptors in porcine kidney

As insufficient information on the endothelin (ET) system in the porcine ki dney is available at present, we investigated renal ET-1 synthesis and ET r eceptors in this species. Because ET specifically affects renal and glomeru lar haemodynamics and distal tubular reabsorption, we studied ET-1 synthesi s in isolated glomeruli and in inner medullary collecting duct (IMCD) cells and preproET-1 mRNA in renal cortex, isolated glomeruli and papillary tiss ue. in addition, we characterized density and properties of ET receptors in membranes from isolated glomeruli and papillary tissue. In contrast to iso lated IMCD cells, which synthesized 120 +/- 11 fmol h(-1) mg(-1) protein of ET-1, no such synthesis was found with isolated glomeruli in our assay sys tem. Nevertheless, with RT-PCR preproET-1 mRNA was clearly present in renal cortex and glomeruli as well as in papillary tissue. Glomerular membranes were found to have ET receptors with B-max of 1.6 +/- 0.2 pmol mg(-1) prote in and K-d of 311 +/- 33 pmol L-1. Using BQ-123 (10(-5) M), a specific bloc ker of ETA receptors, we found that 58% of total receptors are ETA receptor s. Thus, presumably 42% are ETB receptors (B-max 0.7 +/- 0.1 pmol mg(-1) pr otein; K-d 429 +/- 110 pmol L-1). Bosentan (10(-5) M), an ETA- and ETB-rece ptor antagonist, blocked all ET receptors in glomerular membranes. Papillar y membranes showed ET receptors with B-max of 2.1 +/- 0.2 pmol mg(-1) prote in and K-d of 137 +/- 11 pmol L-1. In the presence of BQ-123 (10(-5) M) we found that all receptors are ETB receptors (B-max 2.3 +/- 0.4 pmol mg(-1) p rotein; K-d 162 +/- 25 pmol L-1). Bosentan (10(-5) M) again blocked all ET receptors in papillary membranes, thus confirming our previous finding that IMCD cells possess high-affinity ETB receptors mediating the diuretic effe cts of ET. Thus, in the porcine kidney the ET system may act in an autocrin e/paracrine manner at the glomerular as well as at the IMCD level.