Functional and metabolic effects of terbutaline and propranolol in fast- and slow-contracting skeletal muscle in vitro

Function and distribution of beta3-adrenoceptors in rat, rabbit and human urinary bladder and external urethral sphincter

1. Activation of beta-adrenoceptors causes relaxation of the urinary bladder and contraction of the external urethral sphincter, which consists of fast-contracting skeletal muscles. A beta2-adrenoceptor agonist, clenbuterol, recently has been developed as a therapeutic drug for the treatment of urinary incontinence, however beta2-adrenoceptor agonists have undesirable effects on cardiac and striated muscle function. 2. In this study, we compared the effects of the beta2-adrenoceptor agonist, clenbuterol and of a novel beta3-adrenoceptor agonist, GS332, on urinary bladder and external urethral sphincter function in rat, rabbit and human. We also determined the distribution of beta3-adrenoceptors in human urinary bladder and external urethral sphincter, using radioligand-binding techniques. 3. Clenbuterol induced marked relaxations in rat, rabbit and human urinary bladder smooth muscles and also induced marked contractions in rat periurethral striated muscles (external urethral sphincter), while GS332 induced marked relaxations in rat and human, but not in rabbit, urinary bladder smooth muscles and induced small contractions in rat periurethral striated muscles. 4. The radioligand binding studies showed presence of beta2- and beta3-adrenoceptors in human urinary bladder, external urethral sphincter and abdominal rectus muscles. The affinities of GS332 were the highest in urinary bladder and the lowest in the skeletal (abdominal rectus) muscles, while the affinities of clenbuterol were similar in urinary bladder, external urethral sphincter and the skeletal (abdominal rectus) muscles. 5. These results suggest that GS332 could, similarly clenbuterol, have a role in the treatment of urinary frequency and urinary incontinence.

Resistance exercise overtraining and overreaching. Neuroendocrine responses

Overtraining is defined as an increase in training volume and/or intensity of exercise resulting in performance decrements. Recovery from this condition often requires many weeks or months. A shorter or less severe variation of overtraining is referred to as overreaching, which is easily recovered from in just a few days. Many structured training programmes utilise phases of overreaching to provide variety of the training stimulus. Much of the scientific literature on overtraining is based on aerobic activities, despite the fact that resistance exercise is a large component of many exercise programmes. Chronic resistance exercise can result in differential responses to overtraining depending on whether either training volume or training intensity is excessive. The neuroendocrine system is a complex physiological entity that can influence many other systems. Neuroendocrine responses to high volume resistance exercise overtraining appear to be somewhat similar to overtraining for aerobic activities. On the other hand, excessive resistance training intensity produces a distinctly different neuroendocrine profile. As a result, some of the neuroendocrine characteristics often suggested as markers of overtraining may not be applicable to some overtraining scenarios. Further research will permit elucidation of the interactions between the neuroendocrine system and other physiological systems in the aetiology of performance decrements from overtraining.

The effects of beta-adrenoceptor activation on contraction in isolated fast- and slow-twitch skeletal muscle fibres of the rat

1. The aim of the experiments was to examined the effects of beta-adrenoceptor activation on twitch and tetanic contractions in fast- and slow-twitch mammalian skeletal muscle fibres. Isometric force was recorded from bundles of intact fibres isolated from the normal and denervated slow-twitch soleus and normal fast-twitch sternomastoid muscles of the rat. 2. Terbutaline (10 microM), a beta 2-adrenoceptor agonist, induced an average 15% potentiation of peak twitch and peak tetanic force in normal soleus fibres and abbreviated twitch and tetanic relaxation. In white- and red-sternomastoid fibres, 10 microM terbutaline potentiated peak twitch force by about 7% and slowed twitch relaxation. 3. The potentiation of twitches and tetani by terbutaline was quantitatively similar in normal and denervated soleus fibres. However, in contrast to the normal soleus, terbutaline slowed twitch relaxation and had no effect on tetanic relaxation in denervated soleus fibres. 4. Adrenaline (10 microM) increased peak tetanic force by about 7% in both normal and denervated soleus fibres. 5. Exposure to (+/-)-propranolol (0.1 microM), a general beta-adrenoceptor blocker, completely abolished the tetanus potentiation by terbutaline. 6. Dibutyryl-cyclic AMP (2 mM) mimicked the effects of 10 microM terbutaline on peak tetanic force and tetanic relaxation in normal and denervated soleus fibres. Dibutyryl-cyclic AMP also potentiated peak twitch force in denervated soleus fibres but only after a brief period of twitch depression: the twitch depression might be due to butyrate. 7. The results suggest that the increase in peak twitch and tetanic force and abbreviation of tetanic relaxation induced by terbutaline depend on the activation of beta-adrenoceptors and a consequent increase in the myoplasmic cyclic AMP concentration.

Some new evidence on antifatigue action of aminophylline on the isolated hemidiaphragm of the rat

1. Aminophylline (cumulative concentrations of 0.036-3.60 mmol/l) produced a concentration-dependent increase in both tension developed (Td) and the maximum rate of rise of tension (dT/dt max) of the isolated hemidiaphragm of the rat both during direct single-pulse and subtetanic stimulation. 2. The repeated series of additions of aminophylline into the bathing medium (the second and the third series) produced even further, more pronounced potentiation of both Td and dT/dt max during subtetanic stimulation only, the potentiation being the strongest after the third series of additions of the drug ("antifatigue effect"). The antifatigue effect of aminophylline was much more pronounced than the antifatigue effect of the equimolar concentrations of caffeine. 3. The presence of intact beta 1-adrenergic receptors seems to be essential for the antifatigue action of aminophylline under our experimental conditions. 4. The antifatigue effect of aminophylline was not affected by reserpine or 6-OHDA pretreatment of rats. 5. In a Ca(2+)-free medium the stimulatory effect of aminophylline on Td and dT/dt max was abolished or depressed (single-pulse and subtetanic stimulation, respectively). After returning the muscle into the medium containing Ca2+, the effect of aminophylline was significantly potentiated during both types of the stimulation. 6. The antifatigue action of aminophylline was preserved even in the presence of nicardipine or its solvent in the bathing medium. 7. In the presence of heparin (which produced a significant depression of both Td and dT/dt max by itself during direct subtetanic stimulation) the stimulatory effects of aminophylline on Td and dT/dt max (the second and third series of additions) were significantly potentiated in comparison with the effects of the first series of additions of aminophylline (with no heparin in the bathing medium). 8. The dose-response curves for the effects of aminophylline in the presence of Ni2+ on Td and dT/dt max during direct single-pulse stimulation were significantly shifted to the right. Ni2+ by itself produced significant and dose-related depression of both Td and dT/dt max during single-pulse and subtetanic stimulation, the subtetanic stimulation being much more sensitive. The antifatigue effect of aminophylline during subtetanic stimulation was preserved in the presence of Ni2+. 9. Our results indicate the important role of the extracellular calcium and the involvement of intact beta 1-adrenergic receptors in the antifatigue action of aminophylline. Also, the potentiating effect of heparin on the antifatigue action of aminophylline is presumably due to the influx of extracellular calcium through L-type Ca2+ channels during subtetanic stimulation. Our results indicate the possibility of the presence of T-type calcium channels (which can be blocked by Ni2+) in the isolated hemidiaphragm of the rat, but they do not seem to be involved in the antifatigue action of aminophylline.

Effects of aminophylline and terbutaline sulfate on the contractility of feline diaphragm

1. The effect of aminophylline and terbutaline sulfate, separately and in combination, on the contractility of the feline diaphragm was investigated. 2. Diaphragmatic contractility increased significantly (P less than 0.05) to 28.3% above control level as long as theophylline plasma levels were maintained between 10-20 mg/l (mean 13.9 +/- 0.8 mg/l). 3. Diaphragmatic contractility showed a significant (P less than 0.05) 41.5% increase after i.v. administration of terbutaline sulfate as a maintenance dose of 1.4 micrograms/kg-hr, and the increase leveled off after 180 min. 4. Administering the two drugs in combination showed a significant (P less than 0.05) 27.3% increase which suggests a non-synergistic effect of aminophylline and terbutaline sulfate on diaphragmatic contractility. 5. By enhancing the contractility of the diaphragm, aminophylline and terbutaline sulfate may improve the clinical status of patients with airway obstructive diseases and help in preventing respiratory muscle fatigue.

Multiple actions of beta-adrenergic agonists on skeletal muscle and adipose tissue

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The effect of forskolin on the isometric contraction of the isolated hemidiaphragm of the rat

The effects of forskolin on the parameters of the isometric contraction alone and in combination with aminophylline and isoprenaline were studied on the isolated hemidiaphragm of the rat during direct electrical stimulation. Forskolin (2.6 - 18.2 mumol l-1) produced a concentration-dependent increase in tension developed (Td) and, to a lesser extent, in the maximum rate of rise in tension (dT/dt max). The dose-response curve for the action of forskolin (2.6 - 18.2 mumol l-1) on Td was shifted to the left in the presence of a standard concentration of aminophylline (0.32 mmol l-1). Forskolin (5.2 mumol l-1) produced a further and significant increase in both Td and dT/dt max in the presence of isoprenaline (0.24 mumol l-1) in the bath. In a calcium-free medium, the effects of forskolin (7.80 and 18.2 mumol l-1) on Td and dT/dt max were significantly weaker than in a medium containing calcium. These data indicate that forskolin increases the isometric contraction of the isolated hemidiaphragm probably by activating the adenosine 3':5'-cyclic monophosphate (cyclic AMP) generating system. These effects are possible only in the presence of calcium.

The influence of catecholamines on Na, K transport in slow- and fast-twitch muscles of the rat

The effects of catecholamines on active sodium and potassium transport was compared in slow- (SOL) and fast-twitch (EDL) skeletal muscles of the rat. Stimulation of active Na+-extrusion and K+-uptake induced by adrenaline (6-30 mumol . l-1) and isoprenaline (1-40 mumol . l-1) was markedly greater in slow- than in fast-twitch muscle. In sodium-preloaded muscles the maximal stimulation of 24Na-efflux induced by adrenaline was about 2-fold higher in SOL than in EDL. Isoprenaline caused a 2.4-fold increase in ouabain-sensitive 86Rb influx in SOL muscle, but failed to alter the ouabain-sensitive influx in EDL. The stimulating action of isoprenaline on 86Rb influx in EDL was due to an increase in the ouabain-insensitive fraction of Rb uptake. The effects of catecholamines of fast- and slow-twitch muscles were probably due to the accumulation of cyclic AMP, however the fact that there were no significant differences between the nucleotides levels in fast- and slow-twitch muscle suggests the participation of other mechanism as well. The results presented suggest that cyclic AMP-induced stimulation of ouabain-insensitive transport of cation in the isolated EDL muscle of the rat is similar to that of barnacle muscle.

Desensitization by terbutaline of beta-adrenoceptors in the guinea-pig soleus muscle: biochemical alterations associated with functional changes

1 The effects of adrenaline and terbutaline on cyclic adenosine 3',5'-monophosphate (cyclic AMP) content, 22Na-efflux, 42K-influx and subtetanic contractions have been assessed in soleus muscles isolated from guniea-pigs which had been maintained on food with or without terbutaline for 5 days. 2 Terbutaline and adrenaline increased cyclic AMP content and suppressed subtetanic contractions, and regression analysis indicates a statistically significant correlation between these two effects (P less than 0.01). 3 In muscles obtained from terbutaline-treated animals, the effects of terbutaline and adrenaline on cyclic AMP content, active Na-K-transport and subtetanic contractions were all considerably suppressed, but insulin stimulated 22Na-efflux and affected subtetanic contractions to the same extent as in the muscles obtained from the control group. 4 The results suggest that terbutaline treatment leads to a reduction in the number of beta 2-adrenoceptors in skeletal muscle or an impairment of their function. 5 The results provide further support for the idea that the effect of adrenaline or insulin on skeletal muscle contractions is the outcome of stimulation of active Na-K-transport.