Different effects of verapamil and low calcium on repetitive contractile activity of frog fatigue-resistant and easily-fatigued muscle fibres.

The effects of low calcium and verapamil on contractility of two muscle fibre types (m. iliofibularis, Rana temporaria) upon different stimulation protocols were been compared. Verapamil (0.02 mmol/l) induced temporal excitation-contraction coupling failure during single tetanic stimulation and enhanced the decline of tetanic force during 30 s repetitive tetanic stimulation in both fatigue-resistant fibres and easily-fatigued fibres. In contrast to verapamil, low extracellular calcium (0.02 mmol/l) only enhanced the decline of tetanic force in fatigue-resistant during repetitive tetanic stimulation but had no effect on easily-fatigued fibres. The effect of verapamil on the decline of tetanic force in fatigue-resistant fibres was more profound in low calcium conditions. Both verapamil and low calcium eliminated twitch facilitation that appeared after prolonged contractile activity in fatigue-resistant fibres. 4mmol/l Ni+2, used as calcium channel antagonist, had effects similar to low calcium medium. It could be concluded that (i) extracellular Ca2+-requirements for excitation-contraction coupling are different in fatigue-resistant and easily-fatigued fibres; (ii) the effects of verapamil on force performance are not entirely dependent upon calcium channel blockade.

Modulation by adrenaline of electrophysiological membrane parameters and contractility in intact and internally perfused single muscle fibres of the crayfish.

The effect of catecholamines on basic membrane characteristics (including labeled ionic fluxes) and contractile parameters was followed in current clamp and voltage clamp conditions in intact muscle fibres and internally perfused muscle fibre segments respectively of the crayfish Astacus fluviatilis; i.e. in muscle fibres which spike and activate tension on calcium principle. Both adrenaline and noradrenaline (6.10(-6) mol/l) facilitated twitch tension induced by graded membrane responses or strontium all-or-none spikes. No effect of isoprenaline was observed. Adrenaline (6.10(-6)-6.10(-5) mol/l) produced an inotropic effect, which appeared with a latency of 2 min and reached its maximum in 5 min. The rates of activation and relaxation of contraction were increased, whilst the latency and the threshold depolarization were decreased. The changes persisted (several tens of min) after washout of adrenaline, depending on concentration and duration of adrenaline application. The resting potential and the strontium spike (Ca2+ replaced with Sr2+) were not influenced and the graded responses were facilitated by adrenaline (from 36.4 +/- 1 mV to 40.0 +/- 2 mV; RP = 77.2 +/- 0.5 mV). Extracellular Ca2+ ions are required for the inotropic effect of adrenaline to occur. The decrease of electrical and contractile responses in nominal calcium-free solutions or after a blockade of Ca2+ influx by Ni2+ ions (1 mmol/l) was relieved by adrenaline. The persistence of inotropic effect of adrenaline was absent, when the extracellular concentration of Ca2+ ions, [Ca2+]0 was decreased from 13.5 to 3.4 mmol/l or the Ni2+ ions were added. The influx of 89Sr2+ ions was decreased in the presence of Ni2+ ions from 24.2 +/- 4.7 pmol.cm-2.s-1 to 11.0 +/- 2.8 pmol.cm-2.s-1, but restored to 20.4 +/- 5.8 pmol.cm-2.s-1 in the presence of adrenaline (6 mumol/l). Adrenaline itself decreased the influx of 89Sr2+ ions, and prolonged the time constant of efflux both in resting and stimulated fibres. The effect of adrenaline is dependent on mobilization of Ca2+ ions from the sarcoplasmic reticulum. First, the inotropic effect of adrenaline was absent in the presence of procaine (blockator of the Ca release channel of the SR), in spite of the increase of the active membrane response (all-or-none procaine action potential); second, adrenaline accelerated the uptake of Ca ions by SR as evidenced by shortening of the restitution processes after caffeine contractures by adrenaline. Membrane calcium currents are increased by adrenaline as a rule; mainly at lower depolarizations (-50 to -20 mV).(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of tension decline in different types of fatigue-resistant skeletal muscle fibres of the frog. Low extracellular calcium effects.

Twitch and tetanic tension have been measured in single skeletal muscle fibres (m.ileofibularis, Rana esculenta and Rana temporaria). On the basis of resistance to fatigue produced by repetitive tetanic stable frequency stimulation with various numbers of stimulation trains, twitch fibers were subdivided in three groups (resembling those as described by Westerblad and Lännergren 1986 in Xenopus), that is fatigue-resistant (FR), moderately fatigued (MF) and easily fatiguing (EF). It was found further that the fibres differ in tetanic tension decline resistance i.e. fatiguability relating to some basic contractile parameters including the amplitude, the rates parameters of twitch and tetanus tension as well as the tetanus/twitch tension ratio. The main differences observed concern: 1) The inability to maintain the maximum tetanic tension plateau (IMT) during single tetanus. IMT was 18 times higher in EF fibres and 4 times higher in MF fibres, respectively, in comparison with FR fibres. IMT is the first parameter to change significantly during repetitive tetanic stimulation. 2) The different fibre types show pronounced differences in twitch contraction and tetanus tension during repetitive tetanic stimulation. There is a conspicuous facilitation of twitch tension during and after cessation of repetitive stimulation in FR fibres; the MF and EF fibres show, on the contrary, a depression of twitches. 3) Recovery to original (prefatigue) values is rapid in FR fibres, but slow, however, in EF fibres. 4) Removal of extracellular Ca2+ intensified the inability to maintain the maximum tetanic tension (IMT) and the tetanic tension decline, especially in fibres with an initial high fatigue-resistance. We assume that the results might be explained by a different refractoriness of transmission between the T-tubules and the sarcoplasmic reticulum in examined fibres and/or by a different dependency of the T-SR transmission on the extracellular calcium ions. A possible cause of the failure may be an intensification of the inactivation process.

The effect of Pb2+ ions on calcium currents and contractility in single muscle fibres of the crayfish.

Muscle fibres of crustaceans represent an useful model for studying the mode of action of substances influencing calcium channels as the membrane generates the active responses on a pure calcium principle and the excitation-contraction link is dependent on external calcium. We followed the effect of Pb2+ ions (1-300 mumol/l) on contractile responses in single muscle fibres of the crayfish evoked by massive or intracellular electrical stimulation, by potassium depolarization and caffeine application, as well as on action potential in single intact muscle fibres and on calcium currents in voltage clamp conditions (vaseline gap) in internally perfused muscle fibre segments. All types of contractile responses, single twitches, tetanus, potassium and caffeine contractures were blocked. The strontium action potential was blocked very effectively by Pb2+ ions. The total calcium currents which can be split by means of Hodgkin-Huxley equations into two components (fast and slow respectively) differing in the rate of activation and inactivation were suppressed after addition of Pb(NO3)2 (50-300 mumol/l). The effect of Pb2+ was concentration and time dependent. At lower concentrations (100 mumol/l) the blocking effect was more pronounced on the fast inactivating Ca current component. The Pb2+ ions prolonged the time constant of inactivation tau h of the slow channel, while leaving that of the fast channel intact.

Differences in distribution of ribonuclease isoenzymes in cytosol and granules in normal human granulocytes and in granulocytes of patients with chronic granulocytic leukemia (CGL).

Distribution of ribonuclease (RNAase), acid phosphatase (acid Ph-ase) and beta glucuronidase (BGU) between the granule, cytosol-soluble and post-granule fractions in normal human granulocytes and in granulocytes of chronic granulocytic leukemia (CGL) was studied. CGL granulocytes were found to display relative RNAase activity 1.2 times higher, relative acid Ph-ase activity 2.5 times higher than normal granulocytes. The granule fraction of CGL granulocytes showed 1.4 times higher relative RNAase activity but 0.87 times lower acid Ph-ase activity and the same BGU activity as normal granulocytes. On the other hand, the supernatant soluble fraction of CGL granulocytes showed 4.4 times higher relative RNAase activity, 1.2 times higher relative acid Ph-ase activity and BGU 2.2 times higher than in cytosol soluble fraction of normal granulocytes. Thus, cytosol soluble fraction of CGL granulocytes show a relative activity of the lysosomal enzymes studied which is remarkably higher than in normal granulocytes. The percentage distribution of RNAase, acid Ph-ase and BGU showed that CGL granulocytes contain only 36% of total RNAase activity versus 46% of that in normal ones. On the other hand, CGL granulocytes in cytosol soluble fraction will contain 48% of total RNAase versus 29% of total RNAase in cytosol of normal granulocytes. The isoenzyme profiles of RNAase of granule fractions were similar in normal and CGL granulocytes, while the RNAase isoenzyme profiles of cytosol fractions were different for normal and CGL granulocytes, indicating that some essential part of CGL granulocyte cytosol RNAase differs from RNAase contained in granules and in cytosol of normal granulocytes.