Effects of Calcium Antagonists on Rat Normal and Skinned Fundus

Calcium chloride (CaCl2) (0·1−25 Mm, in K+-depolarized tissue), KCl (10−112 Mm) and acetylcholine (1 × 10−9 m−1 Mm) produced concentration-dependent contractions of rat isolated fundus. Verapamil (0·01−100 μm), cinnarizine (1−100 μm), trifluoperazine (10−500 μm) and dantrolene (50−250 μm) each produced a concentration-related rightward and downward shift of the log concentration-effect curve for CaCl2. The rank order of potencies of these antagonists, measured as the IC50 against Ca2+ (25 Mm)-induced contraction of depolarized fundus, was verapamil (2.5 μm) > cinnarizine (8·7 μm) > trifluoperazine (85·1 μm) > dantrolene (> 250 μm). Cinnarizine (0·5 Mm) and trifluoperazine (0·5 Mm), but neither verapamil nor dantrolene depressed Ca2+ (20 μm)-evoked contraction of rat skinned fundus preparations. In intact preparations of rat fundus, verapamil had greater inhibitory effects on contractions produced by KCl than against those elicited by acetylcholine while trifluoperazine depressed to the same extent the responses to these two spasmogens. Dantrolene was without effect on contractions elicited by KCl or acetylcholine. Cinnarizine inhibited acetylcholine-induced responses but enhanced contractions to KCl. Augmentation of KCl-induced responses by cinnarizine is resistant to verapamil (1 μm). This enhancing effect of cinnarizine was not observed for KCl-induced contraction of guinea-pig fundus or rat gastro-oesophageal sphincter. In the rat fundus, cinnarizine (1−100 μm) produced an additional and concentration-related contraction when added on the plateau contraction to KCl (100 Mm). The enhancing effect and the direct contraction produced by cinnarizine are at least partly dependent on extracellular Ca2+. It is concluded that distinct differences exist between the calcium antagonists examined. The action of verapamil is restricted to the plasmalemma whereas cinnarizine and trifluoperazine also act on the intracellular contractile machinery. Dantrolene is scarcely effective as a calcium antagonist in rat fundus.

Effects of alkylxanthines on contractility of diaphragm fibres isolated from normal and sensitized guinea-pigs

This study investigates the effects of alkylxanthines on twitch tension generated by electrical stimulation (supramaximal pulses, 0.2 ms duration, 1 Hz) of diaphragm muscle fibres isolated from normal and actively-sensitized guinea-pigs. Caffeine, theophylline and theobromine increased, in a concentration-dependent manner (50-500 microM), twitch tension in normal and sensitized diaphragm. Caffeine (500 microM) enhanced contractility to a greater extent than theophylline or theobromine. Twitch potentiation by caffeine (500 microM) was significantly greater in sensitized diaphragm. Verapamil (0.1-100 microM) did not alter twitch contractions in the absence or presence of alkylxanthines in normal or sensitized strips. Dantrolene (0.01-100 microM) depressed, in a concentration-dependent fashion, twitch contractions of normal and sensitized diaphragm. The inhibitory concentration 50% (expressed as -log IC50) was 6.78 +/- 0.13 in normal tissues and 6.15 +/- 0.11 in sensitized tissues (n = 6 in each group; P < 0.05). Exposure to Ca(2+)-free, EGTA (0.1 mM)-containing medium, depressed twitch contraction of normal diaphragm to a lesser extent than that of sensitized diaphragm. Methylxanthines reversed depression of twitch contractions produced by exposure to dantrolene (5 microM) or a Ca(2+)-free medium. Adenosine (1-1000 microM) was without effect whereas enprofylline (50-500 microM) enhanced diaphragm contractility in normal tissues. This indicates that blockade of adenosine receptors is not involved in the inotropic effect of alkylxanthines in guinea-pig diaphragm. Results from this study suggest that alkylxanthines enhance diaphragm contractility in the guinea-pig by releasing intracellular Ca2+ and promoting extracellular Ca2+ entry through verapamil-insensitive pathways. An alteration of Ca2+ movements and stores may be present in the sensitized diaphragm.

Responses to histamine and selective H2-receptor agonists in lung parenchymal strips from normal and sensitized guinea-pigs

Histamine produces concentration-dependent contractions of lung parenchyma strips obtained from normal and sensitized guinea-pigs. The responsiveness of the sensitized lung strips to histamine was significantly increased compared to normal tissues. Clemizole (0.1 microM) was equally effective as an H1-antagonist in normal (dose-ratio 9.12) and sensitized (dose-ratio 9.77) tissues. The concentration-response curves to histamine were displaced to the left by cimetidine (0.1 microM to 0.1 mM) with similar dose-ratios in normal and sensitized tissues. Cimetidine enhanced maximal responses to histamine only in normal lung strips. The effects of submaximal equieffective concentrations of histamine were augmented to the same extent by cimetidine (0.1 mM) in normal and sensitized tissues. The responses to histamine were not modified by indomethacin (5 microM). The responsiveness and sensitivity of sensitized lung strips to isoprenaline, impromidine, 4-methyl-histamine and dimaprit were not different from those of normal tissues. Cimetidine yielded, as antagonist of dimaprit, similar pA2 values in normal and sensitized tissues. In conclusion, there is no experimental evidence in favour of the existence of an impairment of H2-receptor activity in sensitized airways. Hyperreactivity to histamine is probably due to differences between normal and sensitized tissues with respect to Ca2+ entry and/or intracellular Ca2+ release in response to H1-receptor activation.