Effects of dantrolene and methylxanthines on the sensory nerve terminal of the frog muscle spindle

Effect of theophylline and aminophylline on transmitter release at the mammalian neuromuscular junction is not mediated by cAMP

1. Theophylline and aminophylline have been widely used as inhibitors of phosphodiesterase when examining the role of cAMP in regulating cell function. In reality, however, these phosphodiesterase inhibitors may have additional sites of action that could complicate the interpretation of the results. These additional sites of action could include antagonism of inhibitory adenosine autoreceptors and release of intracellular calcium. The purpose of the present study was to determine which of the above three is the primary mechanism by which theophylline and aminophylline affect transmitter release at the mammalian neuromuscular junction. 2. Quantal release measurements were made using intracellular recording techniques. A variety of drugs were used to elucidate this pathway. Isoproterenol, an adenylate cyclase activator, was first used to establish the effect of enhanced levels of cAMP. Theophylline application on its own or in the presence of a drug combination that blocked the adenosine receptor and phosphodiesterase pathways caused significant release depression, opposite to what is expected if it was functioning to enhance cAMP levels. However, when applied in the presence of a drug combination that blocked the adenosine receptor, phosphodiesterase and intracellular ryanodine calcium pathways, theophylline was unable to depress release. Therefore, it was concluded that the major mechanism of action of theophylline is depression of transmitter release by causing the release of intracellular calcium. 3. Aminophylline application alone resulted in a significant enhancement of release. However, when coupled with an adenosine receptor blocker, the ability of aminophylline to enhance transmitter release was blocked, suggesting that its dominant mechanism of action is adenosine receptor inhibition. 4. Taken together, these results indicate that the use of theophylline and aminophylline is inappropriate when examining the role of cAMP at the mammalian neuromuscular junction.

Intramembrane structure of the sensory axon terminals in bullfrog muscle spindles

Much physiologic and morphologic research has been done into the sensory mechanism of the frog muscle spindle. However, no freeze-fracture study has described in detail the shape and intramembrane structure of the nonmyelinated sensory axon terminals of the frog muscle spindle. In this study, muscle spindles were isolated from the red part of bullfrog semitendinous muscles. Chemically fixed spindles were subjected to freeze fracturing. The sensory axon endings were reconstructed, and the size and density of intramembrane particles (IMPs) were measured along the sensory nerve endings. The axon terminals had four distinctive parts: parent trunks (>0.5 microm in diameter), primary branches (0.15-0.5 microm), terminal branches (<0.1 pm), and varicosities (0.02-0.5 microm). IMPs ranged from 5 nm to 21 nm in diameter and were present in the intramembrane space of the plasma membrane all throughout the nonmyelinated sensory nerve endings. Mean IMP sizes in the protoplasmic face (PF) and the external face (EF), respectively, were 8.1 nm and 8.4 nm in the parent trunks, 8.8 nm and 8.8 nm in the primary branches, 9.4 nm and 9.0 nm in the varicosities, and 8.7 nm and 8.7 nm in the terminal branches. Mean IMP size in the PF was smallest in the parent trunk and largest in the varicosity. Mean IMP densities (numbers of IMPs per microm2) in the PF and the EF, respectively, were 2,500 and 700 in the parent trunks, 2,200 and 500 in the primary branches, 1,700 and 400 in the varicosities, and 1,000 and 300 in the terminal branches. Density decreased with the tapering of the axon terminal, with IMPs distributed evenly in the PF and the EF. The characteristic intramembrane structure of sensory nerve endings is discussed.

Pre- and postjunctional actions of purine and xanthine compounds in the guinea-pig caecum circular muscle

1. The sucrose-gap technique was used to study pre- and postjunctional actions of P1-purinoceptor and P2-purinoceptor agonists and a range of xanthine derivatives in the guinea-pig caecum circular muscle. 2. Adenosine, 2-chloroadenosine (2-ClAd), ATP and alpha,beta-methylene ATP all caused concentration-dependent hyperpolarization of the smooth muscle membrane with a rank order of potency of 2-ClAd greater than alpha,beta-methylene ATP greater than adenosine. 3. The xanthine derivatives caffeine, theophylline, 8-phenyltheophylline and 1,3-dipropyl-8-(2-amino-4-chlorophenyl) xanthine (PACPX) at submicromolar concentrations evoked depolarization of the smooth muscle membrane. At higher concentrations, all these compounds and enprofylline caused concentration-dependent hyperpolarization. 4. All the purine compounds tested caused a reduction in the amplitude of the non-adrenergic, non-cholinergic inhibitory junction potential (i.j.p.). For the P1-purinoceptor agonists adenosine and 2-ClAd this was almost entirely a prejunctional effect. For the P2-purinoceptor agonists this was mostly a postjunctional effect because both ATP and alpha,beta-methylene ATP caused significantly greater increases in the conductance of the smooth muscle membrane than did adenosine or 2-ClAd. 5. All the xanthine compounds tested (up to 100 microM), except enprofylline, were capable of increasing the amplitude of the i.j.p. At millimolar concentrations both caffeine and theophylline could reduce the i.j.p. amplitude. 6. It is concluded that there are inhibitory prejunctional P1-purinoceptors on the i.j.p.-producing neurones in the guinea-pig caecum circular muscle and that, of the xanthine derivatives tested, none of them would be suitable to use as a P1-purinoceptor antagonist in this preparation because of their own direct effects.

Effects of cyclic nucleotides and calcium on transduction and encoding processes in frog muscle spindle

In decapsulated muscle spindles, application of dibutyryl cyclic AMP (d-cAMP, 0.1-10 mM) and forskolin (10-100 microM) increased the rate of spontaneous discharges and decreased the responsiveness to stretch. Addition of 2-5 mM CaCl2 or 0.5-2 mM BaCl2 to the above drugs prevented the deterioration of the responsiveness to stretch and the increase in the rate of spontaneous discharges. Similar changes in the afferent discharges were observed with application of isobutylmethyl-xanthine (IBMX, 0.5-2 mM) or carbonyl cyanide chlorophenylhydrazone (CCCP, 1-10 microM). Application of 0.1-1 mM quercetin resulted in a prolonged increase in the discharge rate, lasting 15-20 s after the release of stretch. These results suggest that cAMP is involved in the regulation of the sensory processes through a modification of intracellular calcium activity.

Modification of sensory-terminal responses and membrane structure of frog muscle spindle by collagenase

A preparation of decapsulated muscle spindles with intact sensory innervation has been developed to allow direct access to the sensory terminal for the application of drugs or to alter the extracellular ionic composition. Muscle spindles were isolated from semitendinosus muscles of the frog Rana catesbeiana, and were incubated in a calcium-free Ringer's solution containing 0.2% collagenase. Following optimal incubation at 34 degrees C for 30 min the response pattern of the spindles during stretch could not be distinguished from that of intact spindles, although the duration of individual afferent spikes was prolonged about 4 times normal. The spikes disappeared immediately after the Ringer's solution was replaced with an isotonic choline chloride solution, in contrast to those of intact spindles which remained for 30 min after the replacement. Electron microscopy showed that the outer and inner capsules were partially disrupted. No significant change was observed in the size or packing density of intramembrane particles in freeze-fracture replicas of spindles decapsulated under optimum conditions. More prolonged treatment with the enzyme resulted in abolition of the static component of the response during stretch, and also in an aggregation of the particles, whose size decreased and packing density increased.

Dantrolene. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic use in malignant hyperthermia, the neuroleptic malignant syndrome and an update of its use in muscle spasticity

Dantrolene sodium acts primarily by affecting calcium flux across the sarcoplasmic reticulum of skeletal muscle. Recently, dantrolene has been used very successfully in the treatment of several rare hypercatabolic syndromes which have previously been associated with high mortality rates. In malignant hyperthermia, where early diagnosis and treatment usually with intravenous dantrolene in association with other supportive measures (and often subsequent dantrolene therapy) is performed, recovery is seen in virtually 100% of patients. There is a rapid resolution of hyperthermia, dysrhythmias, muscle rigidity, tachycardia, hypercapnia, mottled or cyanotic skin, and metabolic acidosis, and a slower normalisation of myoglobinuria and elevated serum creatine phosphokinase levels. In patients with family history or previous episodes of malignant hyperthermia, prophylactic treatment with dantrolene prior to anaesthesia prevents the syndrome occurring in most cases. Where malignant hyperthermia has developed patients have been successfully treated with further dantrolene therapy. Dantrolene has also been used successfully in the treatment of a few cases of heat stroke and the neuroleptic malignant syndrome--both of which have many similarities to malignant hyperthermia. Dantrolene is well established in the treatment of patients with muscle spasticity where it generally improves at least some of the components of spasticity (i.e. hyper/hypotonia, clonus, muscle cramps and spasms, resistance to stretch and flexor reflexes, articular movement, neurological and motor functions and urinary control). However, in some patients, particularly those with multiple sclerosis, dantrolene may not be effective, and in many cases muscular strength may diminish. Long term dantrolene therapy has been associated with hepatic toxicity and may cause problems in patients treated for disorders of muscle spasticity. Thus, dantrolene offers a unique advance in the therapy available for the treatment of hypercatabolic disorders and is also useful in the treatment of muscle spasticity of various aetiology.