The potential antidiabetic activity of some alpha-2 adrenoceptor antagonists

The effect of alpha-2 adrenoceptor antagonists, yohimbine and efaroxan, on the plasma glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment with either yohimbine or efaroxan potentiated glucose-induced insulin release in non-diabetic control rats and produced an improvement of the oral glucose tolerance and potentiated glucose-induced insulin release in type-II but not in type-I diabetic rats. Treatment with either yohimbine or efaroxan reduced the plasma glucose level and increased the plasma insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of efaroxan were more marked. Pretreatment of non-diabetic control and type-II diabetic rats with either yohimbine or efaroxan inhibited clonidine-induced hyperglycaemia and suppressed or reversed clonidine-induced hypoinsulinaemia. Also, pretreatment of these animals with either yohimbine or efaroxan enhanced the hypoglycaemic and insulinotropic effects of glibenclamide. The combination of glibenclamide and efaroxan led to a synergistic increase in insulin secretion, while that of glibenclamide and yohimbine led to an additive increase. The hyperglycaemic effect of diazoxide in non-diabetic control and type-II diabetic rats was inhibited by pretreatment with either yohimbine or efaroxan. The hypoinsulinaemic effect of diazoxide in these animals was antagonized and reversed by pretreatment with yohimbine and efaroxan, respectively. In type-I diabetic rats, there was no change in the plasma glucose and insulin levels induced by the treatment of animals with each of clonidine or diazoxide alone or in combination with either yohimbine or efaroxan. Glibenclamide produced a slight decrease in the plasma glucose level of type-I diabetic rats, at the end of the 120 min period of investigation but there was no change in the plasma insulin level. Pretreatment of these animals with either yohimbine or efaroxan produced no change in glibenclamide effects. Additionally, bath application of efaroxan or glibenclamide inhibited the relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips, while the application of yohimbine produced insignificant changes. The combination of glibenclamide and efaroxan led to complete inhibition of the relaxant effects of different concentrations of diazoxide, while that of glibenclamide and yohimbine did not produce such an effect. It is concluded that yohimbine, via blockade of postsynaptic alpha-2 adrenoceptors, and efaroxan, via blockade of postsynaptic alpha-2 adrenoceptors and adenosine triphosphate-sensitive potassium channels in the pancreatic beta-cell membrane, produce insulinotropic and subsequent hypoglycaemic effects.

The myoneural effects of lithium chloride on the nerve-muscle preparations of rats. Role of adenosine triphosphate-sensitive potassium channels

The potential effects of lithium chloride on the neuromuscular transmission and muscular contraction were studied using in vitro and in vivo nerve-muscle preparations of rats. Addition of lithium chloride to the fluid bathing the isolated rat diaphragm produced a concentration-dependent inhibition of diaphragmatic contractions elicited by either indirect or direct electrical stimulation. The threshold concentrations were 1 mmol l(-1)and 3 mmol l(-1), respectively. Similarly, the intravenous administration of lithium chloride as bolus injections, produced a dose-dependent progressive inhibition of the indirectly- and directly-induced gastrocnemius muscle contractions during the 2-h period of investigation. The indirectly-induced contractions were much more sensitive to the inhibitory effect of lithium chloride than directly-induced contractions. Also, lithium chloride was found to be capable of enhancing the paralysis of the indirectly stimulated rat diaphragm in vitro and gastrocnemius muscle in vivo induced by either pipecuronium or succinylcholine. The combination of lithium chloride and pipecuronium led to a synergistic inhibition of the neuromuscular transmission, while the combination of lithium chloride and succinylcholine led to additive inhibition. Pretreatment with lithium chloride at the threshold concentrations enhanced the inhibitory effects of verapamil on diaphragmatic contractions elicited either indirectly or directly. The inhibitory effects of verapamil on the indirectly- and directly-induced rat gastrocnemius muscle contractions were potentiated by lithium chloride administration to rats. Glibenclamide was found to be capable of inhibiting the relaxant effects of lithium chloride on the indirectly- and directly-elicited contractions of rat diaphragm in vitro and rat gastrocnemius muscle in vivo, in a concentration- and dose-dependent manner, respectively. Doubling the concentration of magnesium in the bathing fluid potentiated the inhibitory effects of the threshold concentrations of lithium chloride on the diaphragmatic contractions induced either indirectly or directly. Pretreatment with 4-aminopyridine or barium chloride inhibited the relaxant effects of lithium chloride on the indirectly- and directly-elicited diaphragmatic contractions. The inhibitory effects of diazoxide on the indirectly-evoked contractions of rat diaphragm in vitro and rat gastrocnemius muscle in vivo were potentiated by lithium chloride. Pretreatment with glibenclamide inhibited markedly the combined effects of lithium chloride and diazoxide on the contractions of the diaphragm and gastrocnemius muscles induced indirectly. Additionally, the intravenous administration of lithium chloride into rats as bolus injections produced a dose-dependent progressive increase in plasma potassium level and a dose-dependent progressive decrease in the intracellular levels of adenosine triphosphate in the sciatic nerve and gastrocnemius muscle. It is concluded that lithium chloride, via activation of adenosine triphosphate- sensitive potassium channels, acts presynaptically to inhibit the neuromuscular transmission and acts at the muscle membrane to inhibit the muscular contraction.

Lithium chloride-induced cardiovascular changes in rabbits are mediated by adenosine triphosphate-sensitive potassium channels

The effect of lithium chloride on the mean arterial blood pressure, heart rate and heart contractility was evaluated in rabbits. The intravenous administration of 50 mg kg-1 lithium chloride as a bolus injection into rabbits produced a progressive decrease in the mean arterial blood pressure, heart rate and heart contractility during the 2-h period of investigation. Pretreatment of animals with 5 mg kg-1 glibenclamide, a potent inhibitor of adenosine triphosphate-sensitive potassium channels, markedly inhibited the cardiovascular changes induced by lithium chloride. Doubling the dose of glibenclamide nearly abolished these effects of lithium chloride. Similarly, lithium chloride produced a concentration- dependent relaxation of noradrenaline-induced contractions in isolated aortic strips of rabbits. This relaxant effect of lithium chloride was inhibited by pretreatment of the aortic strips with glibenclamide. Doubling the concentration of glibenclamide in the bathing fluid nearly abolished the effect of lithium. Diazoxide and verapamil potentiated the relaxant effect of lithium chloride on the isolated noradrenaline-contracted aortic strips. Pretreatment with glibenclamide markedly reversed the effect of diazoxide but not that of verapamil. The intravenous administration of lithium was also found to be capable of increasing the plasma potassium level and of decreasing the intracellular levels of adenosine triphosphate in cardiac and vascular tissues in a time-dependent manner. The plasma sodium and calcium levels were not changed. These results provide evidence that the hypotensive and cardiac depressant effects of lithium chloride are mediated by activation of adenosine triphosphate-sensitive potassium channels.

Calcitonin gene-related peptide potentiates nicotinic acetylcholine receptor-operated slow Ca2+ mobilization at mouse muscle endplates

1. The involvement of calcitonin gene-related peptide (CGRP) in the non-contractile slow Ca2+ mobilization induced by prolonged nicotinic stimulation was investigated by measurement of [Ca2+], levels in mouse single muscle cells (flexor digitorum brevis; FDB) loaded with a Ca2+ indicator fluo-3 using confocal laser scanning microscopy. 2. CGRP (3-30 nM) potentiated acetylcholine (ACh, 1 microM)-elicited slow Ca2+ mobilization in a concentration-dependent manner. 3. The potentiation by CGRP of the slow Ca2+ component was greatly depressed by a competitive nicotinic antagonist (+)-tubocurarine (5 microM). The Ca2+ channel blocker nitrendipine (1 microM) affected neither ACh responses nor the CGRP potentiation. 4. The slow Ca2+ component was completely abolished by reducing [Ca2+]0 from 2.5 to 0.25 mM whereas the fast component was not affected. The CGRP-induced potentiation of slow Ca2+ signal was also depressed by decreasing [Ca2+]0. 5. Isoproterenol (30 microM) and 8-bromo-adenosine 3',5'-cyclic monophosphate (1 mM) potentiated the ACh-elicited slow Ca2+ response. The potentiation by CGRP of the slow Ca2+ component was completely abolished by a protein kinase-A inhibitor H-89 (1 microM). 6. These findings indicate that CGRP potentiates the nicotinic ACh receptor-operated slow Ca2+ signal via the activation of protein kinase-A system at the skeletal muscle endplates.

The myoneural effects of propofol emulsion (Diprivan) on the nerve-muscle preparations of rats

The potential effects of propofol emulsion (Diprivan) on the neuromuscular transmission and muscular contraction were studied using in vitro and in vivo nerve-muscle preparations of rats. The contractions of the isolated rat diaphragm elicited by either indirect or direct electrical stimulation were inhibited by propofol emulsion at threshold concentrations of 42 and 112 mumol l-1, respectively. Similarly, the gastrocnemius muscle contractions induced by either indirect or direct electrical stimulation in vivo were inhibited by propofol emulsion administration as a bolus injection of 2.5 mg kg-1 intravenously, followed by intravenous infusion of 150 micrograms kg-1 min-1 for 1 h into rats. The inhibitory effects of propofol in both preparations were greater with indirect rather than direct stimulation. Propofol emulsion was found to be capable of enhancing the paralysis of the indirectly stimulated rat diaphragm in vitro and gastrocnemius muscle in vivo induced by either pipecuronium or succinylcholine. The combination of propofol and pipecuronium led to a synergistic inhibition of the neuromuscular transmission, while the combination of propofol and succinylcholine led to additive inhibition. Pretreatment with propofol emulsion at these threshold concentrations markedly inhibited the stimulant effects of aminophylline and digoxin on the indirectly and directly induced diaphragmatic contractions. Also, the enhancement effects of aminophylline on the indirectly and directly and of digoxin on indirectly induced rat gastrocnemius muscle contractions were markedly inhibited by propofol emulsion administration to rats. Pretreatment with propofol emulsion at the threshold concentrations enhanced the inhibitory effects of verapamil on diaphragmatic contractions elicited either indirectly or directly and enhanced the inhibitory effect of adenosine on the contractions elicited indirectly. Similarly, the inhibitory effects of verapamil on the indirectly and directly and of adenosine on indirectly induced rat gastrocnemius muscle contractions were markedly potentiated by propofol emulsion administration to rats. In addition, doubling the concentration of calcium in the bathing fluid produced no change in the inhibitory effects of propofol emulsion on either indirectly or directly elicited diaphragmatic contractions, while doubling the concentration of external magnesium potentiated the propofol effects. Pretreatment with 4-aminopyridine suppressed the inhibitory effects of propofol emulsion on diaphragmatic contractions elicited either indirectly or directly. These results suggest that propofol acts presynaptically to inhibit the neuromuscular transmission and acts at the muscle membrane to inhibit the muscular contraction.

Adenosine for reversal of hemorrhagic shock in rabbits

The potential use of adenosine in the treatment of hemorrhagic shock was evaluated in rabbits. Hemorrhagic shock was induced by bleeding the animals to a mean arterial blood pressure of 30-35 mmHg that was maintained for 2 hr. The intravenous infusion of 300 micrograms/kg/min adenosine for 1 hr, after reinfusion of the shed blood, was found to be capable of increasing the survival rate of rabbits subjected to hemorrhagic shock. In shocked rabbits, adenosine profoundly improved the postreinfusion depressed contractility of the heart, but it produced a decrease in the mean arterial blood pressure and heart rate. In the same animals, the plasma concentrations of glucose, lactate and inorganic phosphate, which were markedly elevated during shock, were returned back toward normal levels by the intravenous infusion of adenosine. Similarly the alteration that occurred in the plasma sodium, potassium and calcium levels during shock was corrected by adenosine. It is consequently concluded that the use of adenosine after shock improves tissue perfusion and enhances the functional recovery of cells by restoring their metabolic machinery and thereby improves the survival rate.