Intracellular mechanisms of verapamil and diltiazem action on striated muscle of the rabbit

Calcium release by diltiazem from isolated sarcoplasmic reticulum of rabbit skeletal muscle

1. The effect of diltiazem on isolated sarcoplasmic reticulum (SR) from rabbit skeletal muscle was studied. To observe calcium movement into and out of the SR, a fluorescent chelate probe technique with chlortetracycline (CTC) as a reagent was employed. 2. Tris-ATP-induced calcium accumulation by the isolated SR was associated with a rise in the CTC fluorescence. The effect of ATP was dose dependent. 3. Diltiazem (6 x 10(-4)M, 2 x 10(-3)M) prevented ATP-induced calcium accumulation by the SR. 4. Addition of EGTA to the media chelates external calcium and caused calcium release that can be reversed by further addition of calcium chloride. Similarly diltiazem caused a rapid release of accumulated calcium from the SR, which is not reversed by the addition of calcium chloride. 5. It seems that the effect of diltiazem may be related to SR membrane-bound calcium being available for release.

Negative inotropic effect of heparin on tension development in rat skinned skeletal muscle fibres

Heparin inhibits inositol trisphosphate receptors, particularly in smooth muscle, but its effect on skeletal muscle is controversial. Our study showed that heparin induced a decrease in the amplitude of 10 mM caffeine-induced contracture in slow and fast saponin-skinned fibres. Moreover, measurements on Triton X-100-skinned fibres in soleus muscle showed that heparin alone decreased maximal Ca2(+)-activated tension and Ca2+ sensitivity of contractile proteins, whereas no significant effect was observed in extensor digitorum longus muscle. However, in the presence of caffeine, heparin decreased maximal Ca2(+)-activated tension in both muscles. It would appear that the heparin-induced decrease in the amplitude of caffeine contracture in rat skeletal muscle was not related to a direct inhibition of Ca2+ release from sarcoplasmic reticulum but to a desensitising effect of heparin and caffeine on myofilaments.

Frequency-dependent effects of Bay K 8644 on tetanic contractions of frog skeletal muscle

1. The effects of the calcium channel agonist, Bay K 8644 (1-100 microM), on tetanic contractions, elicited by the stimulation frequencies of 100 Hz, 50 Hz and 25 Hz for 2 s, were investigated on frog skeletal muscle fibers. 2. Although the area under the tetanic force versus time curve was greatly reduced at the stimulation frequency of 100 Hz, this effect was significantly reversed at the lower stimulation frequencies of 50 and 25 Hz, at all concentrations tested. 3. During the intracellular recordings, it was revealed that the sodium action potentials elicited with the stimulation frequency of 100 Hz for 2 s were significantly blocked. 4. Similar to mechanical recordings, the blockade of repetitively elicited action potentials was also significantly reversed at lower stimulation frequencies of 50 and 25 Hz for all concentrations of Bay K 8644 tested. 5. In conclusion, the results indicate that Bay K 8644 depresses both tetanic contractions and action potentials in a frequency-dependent manner in frog skeletal muscle fibers.

Effect of gallopamil on excitation-contraction coupling

1. Investigations performed in skeletal muscle have suggested that phenylalkylamine calcium antagonists, particularly gallopamil, affect excitation-contraction coupling independently of their effect on the sarcolemmal calcium current. 2. Sarcoplasmic reticulum and single channel studies have provided evidence that phenylalkylamine calcium antagonists inhibit calcium release through the sarcoplasmic reticulum calcium channel/ryanodine receptor. This action has not been observed with dihydropyridine calcium antagonists. 3. Binding experiments have confirmed the existence of intracellular binding sites for phenylalkylamines, and have shown that gallopamil interferes with the binding of ryanodine to its low affinity sites. 4. The dose-response relationship for the effect of gallopamil on excitation-contraction coupling has not been definitely established. However, there is evidence that gallopamil may be effective at concentrations that are close to the therapeutic range.

Effects of cyclopiazonic acid on Ca(2+)-activated tension production in skinned skeletal muscle fibres of the ferret

The effect of cyclopiazonic acid, an inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, was investigated on force generation by the contractile apparatus in Triton-skinned fibres from extensor digitorum longus and soleus. Concentrations of cyclopiazonic acid lower than 10 microM were without effect on Ca(2+)-activated tension in both types of muscles. In contrast, in soleus, cyclopiazonic acid (20, 50, 100 microM) was found to shift reversibly the relation-tension pCa (-log[Ca2+]) towards lower free Ca2+ and to decrease maximal Ca(2+)-activated tension, in a dose-dependent manner. In extensor digitorum longus, the Ca2+ sensitivity was significantly increased only at a high cyclopiazonic acid concentration (100 microM) and for all the concentrations tested between 5 to 100 microM maximal Ca(2+)-activated tension was unchanged. These results suggest that cyclopiazonic acid has a direct effect on contractile proteins, in a dose-dependent manner. Ca2+ sensitivity and Ca(2+)-activated maximal tension of the contractile apparatus were differentially affected in fast- (extensor digitorum longus) and slow-twitch (soleus) fibres from ferret skeletal muscle.

Organic calcium channel antagonists provoke acetylcholine receptor autodesensitization on train stimulation of motor nerve

The effects of nicardipine and other organic Ca2(+)-channel antagonists on the responses induced by indirect train stimulation (3 s, 50-100 Hz) were studied in mouse phrenic nerve diaphragm preparations. Nicardipine at 1-10 microM, which alone did not affect single or tetanic contractions or the amplitude of evoked endplate potentials and spontaneous miniature endplate potentials, caused tetanic contraction to fade completely in the presence of 0.3 microM neostigmine or 50 microM diisopropylfluorophosphate. In combination with these anticholinesterases, nicardipine caused a severe run-down and shortening of endplate potentials in 1-2 s. This effect on endplate potentials was dependent on stimulus frequency and on extracellular Ca2+. The effect was accelerated by intracellular injection of Ca2+, but retarded by injection of EGTA. The amplitudes of miniature endplate potentials and the evoked endplate depolarization were also depressed during repetitive stimulation. On termination of repetitive stimulation, all postsynaptic responses, including evoked endplate potentials, miniature endplate potentials and single twitches, recovered to pre-train level in 3-10 s. These results suggest that the postsynaptic nicotinic receptors had lost the functional activity during repetitive stimulation. The time-courses of the aforementioned changes initiated by repetitive stimulation were similar to the fast phase of desensitization induced by acetylcholine. The irreversible action of alpha-bungarotoxin on acetylcholine receptor was attenuated in the presence of nicardipine and neostigmine if repetitive stimulation was applied. The same effects were observed with other organic Ca2(+)-channel antagonists (diltiazem, verapamil and nifedipine) as well as agonist (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyr idine- 5-carboxylate, BAY K8644), but not with Mn2+, theophylline or caffeine. It is inferred that organic Ca2(+)-channel antagonists interact directly with acetylcholine receptor ion channel, enhance its autodesensitization liability and thus cause extinction of endplate potentials on repetitive stimulation.