TIGHT-JUNCTION TIGHTNESS OF NECTURUS GALL-BLADDER EPITHELIUM IS NOT REGULATED BY CAMP OR INTRACELLULAR CA2+ .1. MICROSCOPIC AND GENERAL ELECTROPHYSIOLOGICAL OBSERVATIONS

Following the publications by Duffey et al. [Nature 294:451 (1981)] an d Palant et al. [Am J Physiol 245:C203 (1983)] it is generally accepte d that tight-junction tightness of Necturus gall bladder epithelium is up-regulated by cAMP-mediated and Ca2+-mediated stimulation. This con clusion was mainly based on observed increases in transepithelial resi stance (R(t)). However, since in leaky epithelia R(t) cannot be simply equated with the tight junction resistance (R(j)), but may include la rge contributions from the lateral space resistance (R(lis)), we asked whether the observed increases in R(t) resulted indeed from R(j) or w hether R(lis) also increased. The experiments were performed on Nectur us gall bladders using forskolin or the Ca2+ ionophore A23187 as stimu lants. Forskolin (2 mu mol/1) had a biphasic effect. In the first 5 mi n R(t) decreased from 128 +/- 13 to 119 +/- 14 Omega cm(2) (P < 0.05, n = 10) which probably reflects stimulation of an apical cell membrane C1(-) conductance (see accompanying paper). Subsequently R(t) increas ed in approximately 30 min to 184 +/- 20 Omega cm(2) and then remained fairly constant. Simultaneously the lateral spaces collapsed. If the spaces were now transiently opened by passing mucosa-positive direct c urrent across the epithelium, R(t) fell transiently to 111 +/- 7 Omega cm(2), but returned gradually to its elevated lever when the spaces c ollapsed again. When the spaces were constantly dilated by a serosa-po sitive hydrostatic pressure of 1 cm H2O, forskolin neither affected th e space width nor increased R(t), and current passage was virtually in effective, although the cells depolarized in response to forskolin as usual. Similar observations were made with A23187 (10 mu mol/1). It co llapsed the spaces and increased R(t) within 15 min from 145 +/- 9 Ome ga cm(2) to 174 +/- 9 Omega cm(2) and both changes completely reverted during passage of mucosa-positive direct current. Together with the i mpedance measurements reported in the accompanying publication, the da ta indicate that the increase in R(t) following application of forskol in or Ca2+ ionophores primarily reflects a rise in R(lis) and that the tight-junction tightness of Necturus gall bladder epithelium is regul ated neither by cAMP nor by an ionophore-induced increase in intracell ular Ca2+.