Summary
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1.
Manganese ions (0.3–2.4 mmol/l, applied as MnCl2) produced a concentration-dependent rapid decrease in force of contraction (F c) of guinea-pig papillary muscles; 0.6 mmol/l diminisched F c by about 50%. The initial decrease in F c was followed by a concentration-dependent positive inotropic effect which, within 60–90 min, led to a F c value of 100% and more above the control (contraction frequency 0.625 Hz).
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2.
The positive inotropic effect of Mn2+ was associated with an increase in the rate of contraction and a marked, concentration-dependent prolongation of the relaxation time (by 80% in the presence of 1.2 mmol/l Mn2+). Concomitantly, the duration of the action potential was slightly shortened.
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3.
The development of the positive inotropic effect of Mn2+ depended on the stimulation frequency. The half-time values for the increase in F c were 27, 11 and 5 min at the frequencies of 0.5, 0.625 and 1 Hz, respectively.
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4.
After washout of Mn2+ (0.6 mmol/l) the negative inotropic effect disappeared rapidly with a half-time of 0.5 min whereas the positive inotropic effect declined slowly (half-time 38 min).
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5.
It is concluded that, in addition to a negative inotropic effect due to an action on the sarcolemmal membrane, Mn2+, following its stimulation-induced uptake, exerts a positive inotropic effect by an intracellular action. Inhibition of sequestration of Ca2+ by intracellularly accumulated Mn2+ probably plays a role in the positive inotropic effect.
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Vierling, W., Reiter, M. An intracellularly induced positive inotrophic effect of manganese in guinea-pig ventricular myocardium. Naunyn-Schmiedeberg's Arch. Pharmacol. 306, 249–253 (1979). https://doi.org/10.1007/BF00507110
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DOI: https://doi.org/10.1007/BF00507110