[Ca2+]i following extrasystoles in guinea-pig trabeculae microinjected with fluo-3 - a comparison with frog skeletal muscle fibres.
Force production of cardiac muscle is highly dependent on the interval between the excitations. The aim was to investigate relations between intracellular calcium ([Ca2+]i) and force when a stimulus protocol, with three extrasystoles (ESs) at various intervals, was used. The relation between [Ca2+]i and force was compared with that in frog skeletal muscle fibre. Fluo-3 was microinjected into thin cardiac trabeculae to monitor [Ca2+]i. During steady-state [Ca2+]i consisted of a rapid rise (phase 1) that lasted until peak dF/dt (rate of force development) and was followed by a slower rise (phase 2) that coincided with the action potential and had a peak after peak force. The decline in [Ca2+]i outlasted the duration of the contraction. As the ES intervals were prolonged, there was a gradual restitution of force and of the amplitude and rate of rise of phase 1 [Ca2+]i. Peak dF/dt was linearly related to the amplitude of phase 1 [Ca2+]i during restitution and potentiation of force. Skeletal muscle fibres were loaded with fluo-3-AM. From [Ca2+]i the amount of calcium bound to troponin ([Ca-T]) as a function of time was estimated. Force production of the skeletal muscle fibre could be predicted from [Ca-T] when the signal was delayed (time constant 36 ms). This finding indicates that the recorded [Ca2+]i in skeletal muscle represents activator calcium. In cardiac muscle probably only phase 1 [Ca2+]i represents activator calcium. Phase 2 [Ca2+]i probably represents calcium entry during the action potential and does not activate the contractile system to any significant extent.
Wohlfart
B
B
Department of Clinical Physiology and Department of Pharmacology, Lund University, Sweden.
D000200:Action Potentials / Q000502:physiology; D000814:Aniline Compounds; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002138:Calibration; D004558:Electric Stimulation; D005456:Fluorescent Dyes; D006168:Guinea Pigs; D008845:Microinjections; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology* / Q000378:metabolism; D009206:Myocardium / Q000166:cytology* / Q000378:metabolism; D011896:Rana temporaria; D013045:Species Specificity; D013599:Systole / Q000502:physiology*; D014336:Troponin / Q000378:metabolism; D014966:Xanthenes
D000814:Aniline Compounds; D005456:Fluorescent Dyes; D014336:Troponin; D014966:Xanthenes; C059715:Fluo-3; D002118:Calcium