Actions of Calcium Channel Blockers on the Beating Rhythm and Membrane Potential of Cultured Chick Embryonic Heart Cell

Actions of Ca channel blockers such as verapamil and nifedipine on the electrical and mechanical activities of cultured cells from chick embryonic ventricles were studied. The action potentials were divided into three types based on their shape and amplitude; a sino-atrial cell type, a Purkinje fiber type and a ventricular cell type. Some cells without spontaneous beating responded to intracellular stimulation with ventricular type action potential. Both verapamil and nifedipine decreased the rate and strength of spontaneous contractions depending on their concentration. These drugs also reduced the height of plateau of ventricular type action potential much more than that of Purkinje fiber type. Nifedipine, at high concentrations, blocked initiation of the upstroke of the action potential from pacemaker potential. The occasional omission of action potentials resulted in a slow beat rate and irregular rhythm. During the recovery phase from nifedipine treatment, the rate and contractions transiently increased. Positive chronotropic and inotropic interventions during recovery could be explained by the increased Ca influx resulting from the increased concentration gradient between inside and outside of the cells

Properties and Distribution of Fast Twitch and Multiinnervated Slow Fibers in Glycerol-Extracted Extraocular Muscles of the Cat Examined by the Laser Diffraction Method

Mammalian extraocular muscles contain multiinnervated slow fibers. To investigate the properties of these slow fibers, laser diffraction patterns were examined at rest and during contraction. Kittens were anesthetized and the retractor bulbi (RB), the inferior oblique (IO), and the lateral rectus (LR) were isolated from the orbita, and they were treated by 50% glycerol at - 20°C for 4-14 days. These muscle fibers were illuminated by a polarized He-Ne laser beam (632.8 nm) and the intensity of the diffracted light was monitored by moving a small photodiode in the direction perpendicular to the diffraction line. The diffraction patterns of a single fiber prepared from the RB exhibited spatially distinct distribution. The diffraction patterns of the LR and IO, however, were not as sharp as those of the RB. The ratio of peak amplitude to the width at 50% peak of the first order diffraction pattern was taken as an index to describe the sharpness quantitatively. It was observed that most of the muscle fibers from the RB showed high peak/width ratio values, whereas many fibers from the IO and LR exhibted low peak/width ratio values. The muscle fibers with high values were possibly due to fast fibers and those with low values to multiinnervated slow fibers. Based on these results, it was estimated that the slow fibers occupied 30-41% in the IO. 0-11% in the global layer and 36-47% in the orbital layer of the LR. In regard to the contraction, the total light intensity of one side of the firsl order line decreased remarkably during contraction. This came from structural changes in myosin filaments. It is expected that the decrease in the light intensity during contraction would be different for the two types of muscle fibers

Effects of Temperature on Arsenazo III Ca2+-transients in Xenopus Skeletal Muscle Fibers

Arsenazo III (AZ), a Ca2+ indicator dye, was injected iontophoretically into Xenopus skeletal muscle fibers. AZ Ca2+-transients (AZ signals) following action potentials were measured by a pair of photomultipliers at the wavelength of 651-721 nm. As the temperature was lowered, the latency of the AZ signal (T1), the time to peak (T2) and the decay time constant (T3) were prolonged. The values of T1, T2 and T3 were about 1.5, 4 and 25 msec at 20-21°C and about 5, 10 and 80 msec at 7°C. An Arrhenius plot for T2 and T3 indicated that the values of Q10 for T2 and for T3 were 1.7 and 2.2. It was suggested that, at low temperatures, the dissociation of Ca from Ca-binding proteins appears to be the rate limiting factor of the falling phase of the AZ signal

The Mode of the Staircase Phenomenon in Relation to Ryanodine Sensitivity in Rat Papillary Muscle, Bullfrog Atrium and Frog Ventricle

Frog ventricular muscle, when repetitively stimulated at an interval of 1 sec, showed a gradual increase in contraction strength; i.e., the positive staircase phenomenon. The tension of rat papillary muscle, on the other hand, was maximum in the first contraction, after which it decreased gradually during repetitive stimulation; the negative staircase phenomenon. The tension of bullfrog atrial muscle was large in the first contraction, but decreased in the next 15 to 20 contractions and then increased in the following ones. The tension was small if the stimulation interval was prolonged in frog ventricular muscle (positive relationship between contraction strength and stimulation interval). This relationship was reversed in rat papillary muscle. The optimum Ca2+ concentrations for maximum tension development at a stimulation interval of 5 sec were 7.2-9.0 mM in frog muscles and 3.6 mM in rat muscles. Ryanodine inhibited the contraction in rat papillary muscle at lower concentration than those in bullfrog atrial muscle, and it had much less effect on frog ventricular muscle. These results indicate that tissue having high sensitivity to ryanodine exhibits the negative staircase phenomenon, a negative relationship between interval and strength, and low dependency on external Ca2+ concentration

Inhibitory Effects of Heavy Metal Ions on Endolymphatic DC Potential and Cochlear Microphonics in the Guinea Pig

Endocochlear DC potential (EP) and cochlear microphonics (CM) were investigated in the guinea pig under the influence of the following heavy metal ions; manganese, nickel, cobalt, cadmium and lanthanum. The scala tympani and scala vestibuli were perfused with control and test solutions. CM decreased gradually to 50-80%, but EP showed no change after perfusion with a solution containing 1 mM of metal ions. At a concentration of 10 mM, EP decreased from +80 mV to +11-+48 mV and CM decreased to 15-55%. These decreases in EP and CM were irreversible, and perfusion of the area with the standard solution for 20 min had no effect