Lindane decreases forskolin-stimulated cyclic AMP accumulation but does not modify Gs in rat enterocytes

Biochemical effects induced by the hexachlorocyclohexanes

The hexachlorocyclohexanes (HCHs) are synthetic compounds that have been widely used for the control of pests. The most common HCH isomers are the α-, β-, δ-, and γ-HCH. Although the have the same chlorine substitution pattern, the spatial orientation of chlorine atoms is different on each one of them, resulting in unique structures that have distinct molecular properties. Humans are exposed to individual HCH isomers through various routes, including ingestion of contaminated water or food, absorbed through the skin or by inhalation, and because of their liposolubility, these chemicals are mostly stored in fat-containing tissues. The isomer-specific spectrum of biochemical actions for these compounds has been wee characterized for different endpoints such as enzyme activation, calcium homeostasis, gap junctional intercellular communication, endocrine disruption, and cancer, among others. The interaction with the GABA reception has been one of the most extensively studied properties of the HCHs. For instance, γ-HCH acts as a GABAA channel blocker, whereas α- and δ-HCH potentiate currents , all working as allosteric modulators of the receptor. The changes in calcium homeostasis elicited by HCHs are both isomer and cell type specific. For example, in neurons, both the δ- and γ-isomers of HCH stimulate Ca²+ influx through different voltage-gated Ca²+ channels. In human neutrophils, α-,δ-, and γ-HCH, but not β-HCH, increase intracellular Ca²+ concentrations. This isomer-dependent behavior is also similar to that observed for phospholipase A2 activation and also correlates with oxidative stress generation. On the other hand, there are several lines of evidence suggesting that HCHs alter genomic integrity, and, therefore, these compounds have been classified as possibly carcinogenic to humans . Finally, HCHs have been reported to be endocrine disrupters. In fact, γ- and β-HCH have been shown to have weak estrogenic activity, and together with the α- and the δ-isomer, also interfere with steroidogenesis. In short, the HCH isomers are good examples of structurally related chemicals, for which the geometrical patterns present in each one of the different conformers create structures that possess specific mechanisms of action and toxicological properties.

The mechanism for lindane-induced inhibition of steroidogenesis in cultured rat Leydig cells

The in vitro effect of the gamma-isomer of hexachlorocyclohexane, lindane, on rat Leydig cell steroidogenesis was studied. Leydig cells from mature male rats were incubated with human chorionic gonadotropin (hCG, 1 IU) for 3 h at 34 degrees C in the presence of different doses of lindane (2-200 microg/ml; 2-200 ppm). Results demonstrate that lindane produces a dose-dependent inhibition of testosterone production in hCG-stimulated Leydig cells. The decreased testosterone synthesis was accompanied with a half-reduced LH/hCG receptor number without any modification in the K(d) value. In addition, lindane also decreased cAMP production. These effects were not due to a detrimental action of lindane on cell viability. Results of this study demonstrate a direct inhibitory action of lindane on testicular steroidogenesis, at least in part, through a reduction in the classical second messenger production involved in this pathway.

A novel aspect of lindane testicular toxicity: in vitro effects on peritubular myoid cells

The in vitro effects of the insecticide lindane have been investigated in rat testis peritubular myoid cells (PMCs). Upon PMC exposure to lindane, polarity increase and decrease of dipole dynamics were seen at the membrane level (EC50 20 microM), leading to a partial dissipation of the membrane intrinsic dipole potential. The initial membrane depolarization was increased by Cl- efflux and limited by Ca(2+)-activated repolarizing currents. Concomitantly, lindane produced an increase in [Ca2+]i (EC50 125 microM) resulting from both Ca2+ release from an inositol 1,4,5-trisphosphate-sensitive intracellular store and a voltage-dependent Ca2+ influx from the extracellular medium. Of particular interest from a toxicologic point of view, insecticide concentrations well below those effective in altering ion homeostasis potently inhibited both [Ca2+]i increase and contraction induced by the natural agonists vasopressin and endothelin-1 (IC50s < 10 microM). These data demonstrate that PMCs are highly susceptible to lindane and suggest that the insecticide may exert testicular toxicity by interfering with hormone-regulated PMC function.

Urinary trypsin inhibitor reduces the release of histamine from rat peritoneal mast cells

We determined the ability of urinary trypsin inhibitor (UTI), which is a Kunitz-type protease inhibitor present in serum and in urine, to inhibit rat peritoneal mast cell (RPMC) mediator release induced by several stimuli. UTI attenuated the immunoglobulin E-mediated release of both preformed (histamine) and newly formed (leukotriene C4) mediators from RPMCs. Inhibition (21%+/-5%) of the anti-IgE-triggered release of histamine was observed after a 30-minute incubation of RPMCs with UTI (5 micromol/L). To investigate the specificity of the UTI effect, we studied the stimulatory activity of phorbol ester (phorbol 12-myristate 13-acetate (PMA)) or calcium ionophore A23187 in control and UTI-treated mast cells. The efficacy of UTI as an inhibitor was dependent on the nature of the stimulus, because histamine release induced by PMA-mediated or calcium ionophore A23187-mediated processes was not inhibited by UTI. A series of structurally distinct protease inhibitors did not inhibit IgE-induced release of mediators from RPMCs. The Kunitz-type protease inhibitors are important in the regulation of RPMC function. In parallel with the UTI-related decrease in anti-IgE stimulatory activity on mediator release, increased microviscosity of membrane lipids could be observed by two independent experiments on fluorescence polarization with diphenylhexatriene (DPH) and on the fluorescence probe fluorescein isothiocyanate-concanavalin A. UTI reduces mediator release by a mechanism-possibly an interruption of the coupling of receptor and effector systems-because UTI acts as an agent to decrease biologic lipid membrane fluidity.

Hexachlorocyclohexanes affect the arachidonic acid release from phosphatidylinositol but not from other phospholipid classes in tubular cell cultures

Gamma- and delta-isomers of hexachlorocyclohexane caused marked decreases in the levels of radioactive phospholipids, and increases in the levels of [3H]arachidonate incorporated into free fatty acids in rat renal tubular cells. The increased radioactivity of free fatty acids arises from the decrease of [3H]arachidonate incorporated into phosphatidylinositol, but not into phosphatidylcholine, phosphatidylserine or phosphatidylethanolamine. This fact suggests that phosphatidylinositol can be broken down to the fatty acid from the sn-2 position and lysophospholipid by a phospholipase activity increased by hexachlorocyclohexanes. The observed specific toxicant action could be achieved in two ways: (a) operating upon a specific phospholipase A2 that acts on phosphatidylinositol, but not on other phospholipids as substrates and/or (b) involving substrate-phospholipase A2 interactions. Interestingly, the observed effect of the delta-isomer was more pronounced than that of the gamma-one.

Role of lindane in membranes. Effects on membrane fluidity and activity of membrane-bound proteins

The influence of lindane (gamma-hexachlorocyclohexane) on fluidity of plasma membranes from rat renal cortical tubules has been investigated. Preincubation with lindane increased membrane fluidity. This effect was accompanied by (i) a decrease in the transport of glucose with regard to the controls and (ii) an inhibition of the beta-adrenergic stimulatory activity upon cyclic AMP accumulation. However, a significant decrease of the membrane fluidity was found when rats were injected with lindane for 12 days. The injection of lindane exerted the opposite effect on the membrane proteins, the glucose transporter and the beta-adrenergic receptor, enhancing the glucose uptake and increasing the isoproterenol-stimulated cycle AMP accumulation. A possible explanation of the difference could involve a resistance to membrane disordering by lindane through a regulatory mechanism that would balance the activity of many lindane-sensitive proteins in insecticide-injected rats.

Lindane affects phosphoinositide turnover through a different mechanism of the phosphatidylinositol synthesis inhibition in rat renal proximal tubule cell culture

The ability of lindane to change the metabolism of inositol phospholipids was investigated using rat renal proximal tubular cell cultures labelled with [3H]inositol. Lindane addition to the culture medium caused labelling of inositol trisphosphate, inositol bisphosphate and inositol tetrakisphosphate to decrease and that of the inositol monophosphate pool to increase. A depletion of radioactivity in phosphatidylinositols was also observed after lindane addition. Most strikingly, the addition of lindane considerably increased the levels of glycerophosphoinositol in a dose-dependent manner. The effect of lindane follows a different pattern from that of bradykinin, and it is suggested to act by stimulating phospholipase A activity(ies).