Mode of inhibitory actions of acute and chronic chloroquine administration on the electrically stimulated mouse diaphragm in vitro

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Abstract
1 The effects of bath applied chloroquine (Chlo) and of acute and chronic Chlo administration on skeletal muscle reactivity to electrical stimulation and to drugs have been studied on mouse hemidiaphragm preparations in vitro. 2 Chlo (0.15–150 μm) produced a concentration-dependent inhibition of twitch and tetanic contractions due to direct muscle stimulation (MS). Acute and chronic administration of Chlo (45 mg kg−1 i.p. daily, for 3–28 days) progressively shifted the concentration-response curve to bath-applied Chlo to the right, with maximum effect occurring from day 14 of Chlo pretreatment. 3 Acute and chronic administration of Chlo decreased the twitch and tetanus tension, raised the minimal fusion frequency (MFR) for tetanic contraction to occur and did not alter the twitch/tetanus tension ratio. Tetanus tension unlike twitch tension was not significantly decreased on day 3. 4 Caffeine (5–500 μm) - and isoprenaline (0.001-0.8 μm)-induced potentiations of twitch contraction were attenuated in a concentration-dependent manner by bath-applied Chlo and by acute and chronic administration of Chlo. Higher concentrations of caffeine (0.1–5 μm) and KCl (10 mM-130 mm) produced contracture of the muscle which was sensitive to inhibition by Chlo (50–150 μm). Moreover, the spike contractions superimposed on caffeine contracture were more sensitive to the inhibitory effect of Chlo than the contracture. 5 The inhibitory effects of dantrolene sodium and (+)-tubocurarine on MS and on indirectly stimulated hemidiaphragm respectively were not significantly altered by acute and chronic administration of Chlo. In contrast, the inhibitory concentration-response curve to procaine was shifted to the right. 6 The inhibitory effect of bath-appled Chlo, or acute and chronic pretreatment on twitch tension (MS) was not significantly antagonized by stepwise increase in extracellular Ca2+ (0.05-2.5 mm). Sodium thiocyanate (1–5 mm) reversed in a concentration-dependent manner the inhibitory effects of Chlo. 7 Complete recovery of twitch contractions occurred after 3 days of stopping daily Chlo administration, with partial recovery to tetanic tension after 28 days and no recovery of MFR. The reactivity of the diaphragm to bath applied Chlo was progressively restored, whereas the tension curve area to caffeine and KCl was still attenuated even 28 days after stopping Chlo pretreatment. 8 It is concluded that acute and chronic Chlo administration results in changes in reactivity of the hemidiaphragm muscle to electrical stimulation and to drugs such that there is a decrease in muscle strength and tolerance to Chlo in vitro. These effects are dependent on its direct inhibitory action on skeletal muscle and may result from interference with Ca2+ mobilization within the muscle.