Differential effects of phenylmethanesulfonyl fluoride (PMSF) on carbachol and potassium stimulated phosphoinositide turnover and contraction in longitudinal smooth muscle of guinea pig ileum

Interaction of organophosphorus compounds with muscarinic receptors in SH-SY5Y human neuroblastoma cells

Human neuroblastoma cells (line SH-SY5Y) were used to examine the interaction of single exposure to organophosphorus compounds (OPs) with muscarinic receptors. In this study, SH-SY5Y cells were exposed for 30 min to concentrations of paraoxon, diisopropyl phosphorofluoridate (DFP), phenyl saligenin cyclic phosphate (PSP), and mipafox (N,N'-diisopropyl phosphorodiamide fluoridate) that ranged between 10(-9) M and 10(-3) M (10(-2) M for mipafox). Ability to interfere with muscarinic receptor binding was determined by change in the binding of the nonspecific antagonist [3H]-N-methylscopolamine (3H-NMS). Concentrations of paraoxon > 0.5 x 10(-3) M and PSP 1 x 10(-3) M significantly inhibited the binding of a saturating concentration of 3H-NMS. Concentrations of > 10(-5) M paraoxon or PSP could significantly inhibit the binding of a half-saturating concentration of 3H-NMS. Studies using specific antagonists for muscarinic subtypes (pirenzepine for M1, AFDX-116 for M2, and 4-DAMP for M3) indicated that SH-SY5Y cells have muscarinic receptors most sensitive to the specific antagonist for the M3 subtype (IC50 of 10(-8) M for 4-DAMP compared to 2.5 x 10(-6) M and 2.7 x 10(-5) M for pirenzepine and AFDX-116, respectively). As M3 receptor stimulation results in formation of inositol phosphates from membrane phosphoinositides the capability of OPs to alter levels of inositol phosphates and agonist-stimulated increases in inositol phosphate formation was examined. Intact cells were prelabeled with [3H]myo-inositol and then incubated for 15 min with the OPs before addition of 10(-5) M to 10(-3) M carbachol. Levels of inositol phosphates were determined as the amount of aqueous soluble radiolabeled product extracted from the reaction mixture. Paraoxon and PSP, but not mipafox or DFP, decreased basal levels of inositol phosphates in a concentration-related manner. This could be overcome in cells stimulated with carbachol, a muscarinic agonist, and with sodium fluoride, which does not act at muscarinic receptors. These results indicate that certain OPs, upon acute exposure, interact with muscarinic receptors, but that they also have effects on levels of inositol phosphates that may be associated with another site of action in SH-SY5Y cells.

The diacylglycerol kinase inhibitor, R 59022, suppresses contractility of intestinal smooth muscle

R 59022, a compound which causes the accumulation of diacylglycerol by inhibition of the enzyme diacylglycerol kinase, was tested for its effect on stimulated motor activity of the longitudinal muscle from the guinea-pig ileum. Motor responses to histamine were very potently inhibited by R 59022 (greater than or equal to 0.03 microM), which confirms its known activity as a histamine receptor antagonist. Contractions elicited by acetylcholine, substance P, or K+ depolarization were also concentration dependently depressed by R 59022 (1-30 microM); further analysis showed that substance P was antagonized in a non-competitive manner. R 59022 was significantly more active to block the tonic than the phasic component of the contractile response to K+ depolarization. The Ca2+-induced activation of the contractile apparatus in chemically skinned muscle strips was depressed by similar concentrations of R 59022 (3-30 microM). These data indicate that R 59022 suppresses the contractile activity of intestinal smooth muscle at an intracellular site of action but it is not yet clear whether this action can be accounted for by the accumulation of diacylglycerol and subsequent stimulation of protein kinase C.

Protein kinase C may regulate the tonic component of intestinal smooth muscle contraction in response to substance P

(1) The study investigated a possible involvement of protein kinase C (PKC) in the substance P-induced contraction of the longitudinal muscle of the guinea-pig isolated ileum. (2) The predominant effect of the PKC activator, phorbol-12,13-dibutyrate (PDB), was to change the time course of the response to substance P. While the initial peak contraction was hardly influenced by PDB, the fading of the contraction was accelerated to an extent that any tonic contraction which normally followed the initial peak response was prevented. This inhibitory effect of PDB on the tonic contraction was immediate in onset and related to its concentration (20-200 nM); responses to half-maximally (2-7 nM) or maximally effective (0.74 microM) concentrations of substance P were affected in the same manner. Tetrodotoxin (0.6 microM) did not alter the effect of PDB. Phorbol-13-monoacetate (2 microM), a phorbol ester which does not stimulate PKC, failed to change the time course of the substance P-induced contraction. (3) The tonic component of half-maximal contractile responses to histamine (0.2-0.4 microM) was also depressed by PDB (0.2 microM) whereas the tonic component of maximal responses to histamine (9 microM) was enhanced. (4) PDB (0.2 microM) reduced desensitization to substance P as judged by the reduction of the peak response to substance P (2-7 nM) following a 10-min exposure to a high concentration of the peptide (0.74 microM). (5) The PKC inhibitor, polymyxin B (0.1-0.3 mM), reduced the peak contractile response to substance P, slowed the fading of the contraction, and antagonized the inhibitory effect of PDB on the tonic contraction.(ABSTRACT TRUNCATED AT 250 WORDS)