LINDANE INHIBITION OF GAP JUNCTIONAL COMMUNICATION IN MYOMETRIAL MYOCYTES IS PARTIALLY DEPENDENT ON PHOSPHOINOSITIDE-GENERATED 2ND MESSENGERS

The ability of environmental contaminants to modulate gap junctional c ommunication between uterine smooth muscle cells is generally unknown, despite recognition that myometrial gap junctions may play a role in synchronizing uterine contractions during parturition. The present stu dy tested the hypothesis that the organochlorine pesticide lindane (ga mma-hexachlorocyclohexane) inhibits gap junctional communication in my ometrial myocytes due to the release of phosphoinositide-dependent sec ond messengers. The effect on gap junctional communication by lindane was tested in cultured rat myometrial smooth muscle cells by monitorin g transfer of the fluorescent dye Lucifer yellow. A rapid, concentrati on-dependent, but reversible inhibition of dye transfer was noted with 4-min exposures, and inhibition was complete with 10 mu M lindane. Li ndane also stimulated the production of the Ca2+-releasing species ino sitol 1,4,5-trisphosphate which peaked at 5 min (100 pmol/mg protein) and remained elevated after a 15-min exposure. To examine the possible inhibitory role of Ca2+ on gap junctions, the Ca2+ ionophore 4-br-A23 187 was used. Although A23187 also inhibited gap junctional communicat ion, inhibition was not complete even at concentrations that appeared cytotoxic (70% inhibition at 2 mu M A23187). Cells were then loaded wi th the Ca2+ chelator BAPTA-AM, which blocked the lindane-induced rise in calcium, and dye transfer experiments with lindane were repeated in Ca2+-free medium. Inhibition of dye transfer was still complete under these conditions, showing that increased intracellular calcium was no t required for lindane-induced inhibition of gap junctional communicat ion. Subsequently, 10 mu M lindane was shown to produce a sustained in crease in protein kinase C (PKC) activity (31, 17, and 15 pmol of PKC peptide phosphorylated/min/mg protein for 2-, 5-, and 10-min exposures , respectively). Known activators of PKC, 12-O-tetradecanoylphorbol 13 -acetate (TPA) and 1,2-dioctanoyl-sn-glycerol, abolished gap junctiona l communication at nanomolar concentrations. Although use of the PKC i nhibitor staurosporine failed to reverse lindane's inhibitory action, depletion of PKC activity through prolonged exposure to TPA partially reversed lindane's effect. This suggests that PKC activation potentiat es but does not solely mediate lindane's inhibitory action on gap junc tional communication. (C) 1995 Academic Press, Inc.