Lindane inhibition of gap junctional communication in myometrial myocytes is partially dependent on phosphoinositide-generated second messengers

The ability of environmental contaminants to modulate gap junctional communication 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 study tested the hypothesis that the organochlorine pesticide lindane (gamma-hexachlorocyclohexane) inhibits gap junctional communication in myometrial myocytes due to the release of phosphoinositide-dependent second messengers. The effect on gap junctional communication by lindane was tested in cultured rat myometrial smooth muscle cells by monitoring transfer of the fluorescent dye Lucifer yellow. A rapid, concentration-dependent, but reversible inhibition of dye transfer was noted with 4-min exposures, and inhibition was complete with 10 micromolar lindane. Lindane also stimulated the production of the Ca2+-releasing species inositol 1,4,5-trisphophate 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-A23187 was used. Although A23187 also inhibited gap junctional communication, inhibition was not complete even at concentrations that appeared cytotoxic (70% inhibition at 2 micromolar A23187). Cells were then loaded with 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 not required for lindane-induced inhibition of gap junctional communication. Subsequently, 10 micromolar lindane was shown to produce a sustained increase 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 junctional communication at nanomolar concentrations. Although use of the PKC inhibitor 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 potentiates but does not solely mediate lindane's inhibitory action on gap junctional communication.