ELECTROPHYSIOLOGICAL PROPERTIES OF DIFFERENT CELL-TYPES IN THE SHARK RECTAL GLAND

Electrophysiological properties of different cell types were studied i n single rectal gland cells of Squalus acanthias by the whole-cell vol tage clamp technique. Based on electrophysiological characteristics an d primary morphological observations (light microscope, X400), three c ell types (named as I, II, and III) were found in isolated fresh cells and two cell types (I and II) in primary cultured cells of the shark rectal gland (SRG). Type I cells had both Cl- (I-Cl and the inwardly r ectifying K+ channel (I-K1). Type II and III cells only had I-Cl. Unde r X400 light microscope granular materials in the cytoplasm were found in Type I and II cells, but not in Type III cells. The data from this study show that 65 % of isolated fresh SRG cells strongly expressed t he K+ channel with much less amount of the Cl- channel and 35% had onl y I-Cl. In sharp contrast, 11% had I-K1 and I-Cl, and 89% had only I-C l in cultured SRG cells. Extracellular application of 10 mu M forskoli n significantly enhanced I-Cl in primary cultured SRG cells. This enha ncement was influenced by intracellular Ca2+ and blocked by 50 mu M Ni 2+. Other compounds, such as vasoactive intestinal peptide (VIP) and 8 -(4-chlorophenylthio)-adenosine 3':5'-cyclic monophosphate (cpt cAMP) also enhanced I-Cl. Interestingly, cAMP and forskolin significantly in hibited I-K1 in cultured and fresh SRG cells. I-K1 was blocked by micr omolar concentrations of Ba2+ and significantly altered by extracellul ar K+ concentrations. The present data suggest that 1) the shark recta l gland contains different cell types which may play various roles in the process of salt secretion; 2) I-Cl and I-K1 in SRG cells are stron gly modulated by cAMP, forskolin, and VIP, as well as Ca2+, K+, and Na + ions.