THE PARIETAL SHEET OF BOWMANS CAPSULE OF RAT RENAL GLOMERULUS - A TARGET OF ENDOTHELIN AND PAF

On the basis of intracellular calcium concentration ([Ca2+](i)) measur ements, we have previously reported that the parietal sheet of Bowman' s capsule was sensitive to cholinergic agonists. The aim of the presen t work was to investigate whether this structure could be also a targe t of endothelin and platelet-activating factor (PAF), since we observe d [Ca2+](i) increases in response to both agonists in the glomerulus, but which were very different from that induced by carbachol. For this purpose, we measured [Ca2+](i) on single microdissected parietal shee ts, using a fura 2 microfluorescence technique and compared the effect s of maximal concentrations of the three agonists (10(-7), 10(-8), and 10(-4) M for endothelin, PAF, and carbachol, respectively) under vari ous experimental conditions. We observed that, like in the glomerulus, endothelin and PAF induced, in the parietal sheet, [Ca2+](i) response s that differed in many respects from those found with carbachol. Thus , in the presence of 2 mM external calcium, 1) endothelin and PAF resp onses spontaneously declined to basal level, whereas a stationary plat eau was observed after a sharp peak of [Ca2+](i) with carbachol; 2) th e magnitude of [Ca2+](i) peak was smaller with endothelin and PAF than with carbachol; and 3) endothelin and PAF, but not carbachol, induced a homologous dose-dependent desensitization. Moreover, in the absence of external calcium, endothelin and PAF responses were smaller than c arbachol response, although all three responses apparently resulted fr om release of calcium ions from the same internal pool. In additional experiments, we observed that, like carbachol, endothelin and PAF cont racted the parietal sheet, which is only composed of myoepithelial cel ls. In conclusion, three agonists, namely carbachol, endothelin, and P AF, are now known to act on the parietal sheet by increasing [Ca2+](i) and inducing contractions.