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The R145G amino acid exchange in the inhibitory subunit (cTnI) of cardiac troponin, which regulates muscle contraction, is related to familial hypertrophic cardiomyopathy. Information on its impact on contractility of adult cardiomyocytes is scarce. We studied shortening of adult rat cardiomyocytes before and during ss-adrenergic stimulation using adenovirus-driven expression of human cTnI-wild type (wt) and cTnI-R145G. Baseline sarcomere shortening was significantly decreased by cTnI-R145G expression. Upon ss-adrenergic stimulation using isoproterenol (ISO), nearly identical amplitudes of shortening were obtained with cells expressing cTnI-R145G and control cardiomyocytes (native and cTnI-wt). However, rates of shortening and relengthening were depressed in cTnI-R145G-expressing cells but were comparable to those of control cells upon addition of forskolin or ISO and ICI118,551. This indicates that cTnI-R145G expression influences the response to ss-adrenergic stimulation dependent on the receptor subtype.

The dependency of ATP utilization and isometric force on [MgATP] was studied in skinned rat trabeculae under normal (pH 7.0) and simulated ischaemic (pH 6.2, 30 mM added Pi) conditions at 20±1 °C. At saturating [Ca2+], mean (±SEM) ATP utilization at 5 mM MgATP (A 0) was 0.48±0.03 mM/s and force (F 0) was 37±2 kN/m2. At 10 µM MgATP under normal conditions ATP utilization decreased gradually to 66±3% of A 0 , and force increased to 169±7% of F 0. Under ischaemic conditions at 10 µM MgATP, ATP utilization decreased from 30±5% to 11±2% of A 0 whereas force increased eightfold from 12±4% to 97±7% of F 0. The [MgATP] at half-maximal ATP utilization (K m) under ischaemic conditions was 21±3 µM. At pH 7.0, K m was estimated to be less than 10 µM. These results show that tension cost decreases markedly with decreasing MgATP. Under ischaemic conditions parallel changes in Ca2+ sensitivity of force and ATP utilization were observed, corresponding to 1.3 pCa units. Reducing [MgATP] from 0.5 to 0.05 mM caused a modest reversal of this change in Ca2+ sensitivity. These changes in Ca2+ sensitivity are consistent with a marked reduction in active force and force-related ATP utilization during ischaemia but are insufficient to explain the ischaemic contracture on the basis of active force development.

We studied the relation between force normalized by dry mass per unit length and the myosin fraction of muscle dry mass. The two tibialis anterior muscles were dissected from 12 frogs (Rana temporaria). Then, from one muscle, two single fast-twitch fibres were isolated. Each fibre was mounted isometrically in Ringer's solution, and electrically stimulated using a standardized protocol. Peak force production, normalized by the fibre's dry mass per unit length, varied by a factor of 1.4. Little variation in normalized force was measured between fibres from the same animal, whereas between animals a significant difference was found (P

Calcium activated isometric tension development was measured in single skinned muscle fibres of the ileofibularis muscle of the frog. The experiments were carried out at 5 degrees C, pH = 6.9, 1 mM free Mg2+ and an ionic strength of 160 mM. A Hill curve was fitted to the isometrically developed tension at different Ca2+ concentrations by means of a non-linear least mean square approximation. At a sarcomere length of 2.15 micron, the Ca2+ concentration for half maximum tension (K) was 1.6 microM. This Ca2+ concentration decreased with increasing sarcomere length; at 2.7 micron, K was 1.1 microM and at 3.1 micron, K was 0.9 microM. Therefore, Ca sensitivity is increased at larger sarcomere lengths. Consequently, the optimal sarcomere length for tension development shifted to larger values when the Ca2+ concentration was lowered. Osmotic compression of the fibre at 2.15 micron by means of 5% Dextran also caused an increase in Ca sensitivity (K was 1.0 microM). At 2.7 micron, addition of 5% Dextran hardly affected the Ca sensitivity. The possible role of the interfilament spacing in the explanation of these results discussed.

Tension responses due to small rapid length changes completed in 50 microseconds were obtained from segments with different length of single fibres of the ileofibularis muscle of the frog. The very early parts of the responses varied with segment length. A simulation of the early parts of the response was carried out by means of a linear model in which the fibre is regarded as a rod of infinitesimally small segments containing undamped elasticity, damped elasticity and mass in series. In the simulation corrections were included for the effects caused by the viscosity and density of the surrounding fluid and for the force transducer characteristics. The results indicate the presence of a very rapid component in the fast recovery with a time constant of 5-15 microseconds. The undamped elasticity of the activated fibres corrected for their passive properties was such that a sudden shortening corresponding to 2.6 nm/half sarcomere would reduce active tension to zero.

Tension responses due to rapid length changes completed in 50 and 150 microseconds were obtained from activated skinned single fibres of the ileofibularis muscle of the frog. The natural frequency of the force transducer was about 50 kHz. The length changes ranged between -1% and +0.5% of the fibre segment length. The sarcomere length was adjusted to 2.15 micron. The temperature was maintained at 2.5 degrees C. The transmission velocity estimated from these recordings obtained on fibre segments with different length was 230 m/s in fully activated segments and 112 m/s in relaxed segments. The initial part of the responses during the length changes consisted of an abrupt change in tension reaching an extreme value T1, which depended on the amplitude as well as the duration of the length change. A partial rapid recovery towards a plateau occurred after the length change. The reciprocal half-time of this recovery increased with the amplitude of the displacement both for stretches as well as releases up to about 5 nm/half sarcomere.