[Ca2+]i following extrasystoles in guinea-pig trabeculae microinjected with fluo-3: A comparison with frog skeletal muscle fibres

• Published in: Acta physiologica Scandinavica 2000-05, Vol.169 (1), p.1-11
• Main Author: WOHLFART, B
• Format: Article
• Language: English
• Subjects: Action Potentials - physiology; Aniline Compounds; Animals; Biological and medical sciences; Calcium - metabolism; Calibration; Electric Stimulation; Fluorescent Dyes; Fundamental and applied biological sciences. Psychology; Guinea Pigs; Heart; Microinjections; Muscle Contraction - physiology; Muscle Fibers, Skeletal - physiology; Muscle, Skeletal - cytology; Muscle, Skeletal - metabolism; Myocardium - cytology; Myocardium - metabolism; Rana temporaria; Species Specificity; Striated muscle. Tendons; Systole - physiology; Troponin - metabolism; Vertebrates: cardiovascular system; Vertebrates: osteoarticular system, musculoskeletal system; Xanthenes
• ISSN: 0001-6772; 1365-201X
• DOI: 10.1046/j.1365-201X.2000.00687.x

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.