Propylene glycol monomethyl ether (PGME) exposure. 2. Identification of products containing PGME, their importance and their use in Switzerland.

OBJECTIVE: In order to identify users of PGME and potential exposures, a chemical registration database maintained in Switzerland was analysed. METHOD: The database contains information on the composition of products (qualitative and quantitative), the field of use, the year of registration and the domain of commercial applications (public or professional). RESULTS: Identification of potential exposures in Switzerland was carried out. Out of a total of 150,000 products, 2334 were found to contain PGME and most contained between 1% and 10% PGME. There was a great increase in the number of products declared between 1983 and 1991. The principal fields of use were in inks, varnishes and paints.

Rabbit aorta: electrical properties and agonist-induced depolarization.

The resting membrane potential of the smooth muscle cells of strips of rabbit aorta varied between -50 to -60 mV. No spontaneous spike discharges were observed. The membrane of the myocytes showed a marked outward rectifying property. The rabbit aorta had cable properties with a space constant (lambda) of 1.54 +/- 0.04 mm. Parallel with a progressive mechanical tension development, noradrenaline and the alpha 1-agonist methoxamine depolarized the membrane in a concentration-dependent manner up to -40 mV. Stimulation of aortic alpha 1-adrenoceptors by noradrenaline reduced the steepness of the current-voltage relationship and diminished the space constant from 1.54 to 0.8 mm, indicating a decrease in membrane resistance. No action potentials were evoked by noradrenaline. The alpha 2-agonist, B-HT 920, produced only a slight contraction and virtually no change in membrane potential. As compared to noradrenaline or methoxamine, angiotensin II was a partial agonist to induce contraction, with an intrinsic activity of 0.6-0.7. The octapeptide depolarized the membrane of the myocytes in a concentration-dependent manner and to a maximal extent similar to that observed for alpha 1-adrenoceptor stimulation. No action potentials appeared with angiotensin II. In contrast to earlier reports, depolarization did occur in the rabbit aorta in response to noradrenaline. The demonstration of depolarization raises the possibility that contraction of this blood vessel occurs through electromechanical and not or not solely through pharmacomechanical coupling when receptors for two important endogenous vasoconstrictors, noradrenaline and angiotensin II, are stimulated.

Vascular smooth muscle: availability of calcium through alpha-adrenoceptor stimulation.

In strip preparations from the rabbit main pulmonary artery, it was not possible to differentiate convincingly between alpha 1- and alpha 2-adrenoceptors, when selective agonists such as methoxamine, St 587, B-HT 920, and clonidine, as well as selective antagonists such as prazosin and yohimbine were used for receptor characterization. Measurements of the membrane potential of the vascular smooth-muscle cells with intracellular glass micropipettes showed similar depolarizations in response to the alpha 1-selective methoxamine and the alpha 2-selective B-HT 920. These observations suggest the occurrence of a uniform alpha-adrenoceptor population in the rabbit main pulmonary artery. However, in contrast to alpha 1-agonists, contractile responses to alpha 2-agonists differed considerably in their dependence on external Ca, as revealed in Ca withdrawal experiments, as well as by the use of Ca antagonists. Furthermore, inactivation of the guanine nucleotide-binding inhibitory protein (Ni protein) by pertussis toxin impaired vasoconstriction, in response to alpha 2-agonists, but left that to alpha 1-agonists unaffected. It is suggested that alpha 1- and alpha 2-agonists induce two distinct conformational changes in an otherwise uniform alpha-adrenoceptor of the rabbit main pulmonary artery. By the mediation of Ni protein, alpha 2-agonists appear to allow influx of Ca into the vascular muscle cells through a type of Ca channel that is unlikely to be controlled by voltage.

Electrical membrane events in response to alpha-adrenoceptor stimulation.

Strips of rabbit main pulmonary artery (RMPA) were used to study the effects of several agonists on tension development and membrane potential of the vascular smooth muscle cells. The following alpha-adrenoceptor agonists were employed: methoxamine and St 587 (alpha 1-selective), B-HT920 (alpha 2-selective) and clonidine, which stimulates preferentially alpha 2-adrenoceptors. By the use of the selective antagonists prazosin and yohimbine it was not possible to differentiate convincingly between alpha 1- and alpha 2-adrenoceptors in the RMPA. Methoxamine and B-HT920 produced depolarization of similar magnitude of the membrane of the vascular smooth muscle cells. In spite of these results, which point to a uniform alpha-adrenoceptor in the RMPA, contractions to alpha 1- and alpha 2-agonists differed in some important aspects. Contractions in response to alpha 2-agonists were highly susceptible to the inhibitory effects of calcium withdrawal and calcium antagonists whereas contractions to alpha 1-agonists were much less so. Reduction of the membrane potential of the vascular cells by K+ at 12 mmol/l had no effect on the concentration-contraction curve of methoxamine but shifted that of B-HT920 to the left. Conversely hyperpolarization of the membrane of the vascular smooth muscle cells by strychnine totally suppressed contraction to B-HT920 and caused only a rightward shift of the concentration-contraction curve of methoxamine and St 587. Interaction of alpha 1- and alpha 2-agonists with an apparently uniform alpha-adrenoceptor induces in the RMPA contraction which seems to be triggered by different membrane processes.

Ca2+ channel modulation by 8-bromocyclic AMP in cultured heart cells.

Modulation of ion channels is of increasing interest as it is an important step in the regulation of cellular functions. We have analysed the effect of 8-bromocyclic AMP on Ca2+ channels in cultured cardiac cells by the patch-clamp method and report here that there was a large increase in the probability of opening of the channels. On the basis of a recently proposed kinetic reaction scheme we suggest that cyclic AMP-dependent phosphorylation of Ca2+ channels primarily promotes the forward rate constants which lead to the open state of a Ca2+ channel during depolarization.

Sodium channels in cultured cardiac cells.

Primary cardiac cell cultures were prepared from the hearts of neonatal rats. The patch-clamp method (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) was applied for studying whole-cell Na+ currents and single-channel Na+ currents, respectively. Whole-cell recordings yielded voltage- and time-dependent Na+ currents which could be blocked by tetrodotoxin. Single-channel Na+ currents were directly compared in cell-attached patches and in inside-out patches. In cell-attached patches the elementary current was about -1 pA at -10 mV and the slope conductance over a 50 mV voltage range was 15.1 +/- 1.6 pS (mean +/- S.D.). Inactivation during depolarization and after conditioning clamp steps, in the steady state, resulted from a reduced opening probability of Na+ channels. In inside-out patches, with identical solutions at both membrane surfaces, there was a large (40-50 mV) shift of channel opening and inactivation kinetics towards more negative potentials. However, for levels of comparable opening probabilities, mean open times of Na+ channels were similar in cell-attached and inside-out patches. Tetrodotoxin (10-20 microM) had no effect on Na+ channels when applied from the inside, but blocked them completely after application to the outside membrane surface.

Ca2+ -activated K+ conductance in internally perfused snail neurons is enhanced by protein phosphorylation.

Depolarizing voltage steps induce inward and outward currents in voltage-clamped, internally perfused neurons from the snail Helix roseneri. Addition of the catalytic subunit of cyclic AMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) to the internal perfusing medium results in an increase in the net outward current, with no apparent effect on the inward current. Catalytic subunit inactivated by 5,5'-dithiobis(2-nitrobenzoic acid) is without effect, indicating that the increase in net outward current results from protein phosphorylation rather than an unspecific effect of protein perfusion. Decreasing the external Ca2+ concentration from 10 to 1 mM eliminates the effect of catalytic subunit, suggesting that Ca2+ plays an important role in this response. This suggestion is supported by the fact that the stimulation by catalytic subunit can be mimicked by increasing the Ca2+ concentration in the internal perfusion medium and can be prevented by intracellular perfusion with 10 mM EGTA. The results are consistent with the hypothesis that cyclic AMP-dependent protein phosphorylation regulates the Ca2+-activated K+ conductance in these cells.

Divalent cations as charge carriers during two functionally different membrane currents in the ciliate Stylonychia.

We have investigated the ability of various divalent cations to carry current through membrane channels in ciliates activated by mechanical stimulation and by membrane depolarization. Both types of channels are identified as Ca2+ channels. Whereas Ba2+ ions and Sr2+ ions can replace Ca2+ ions as charge carriers during either kind of current, Mg2+ ions can carry the current evoked by mechanical stimulation but not the current elicited by membrane depolarization. Mn2+ ions did not carry either of these currents, and they reduced the currents carried by Ca2+ ions. Our experiments suggest that the mechano-sensitive and the voltage-sensitive channels exhibit different selectivities for divalent cations.