Ouabain hypoglycemia: insulin mediation.

The fall in blood sugar occurring during infusion of ouabain (1 microgram per kilogram per minute for a 60-minute period) in dogs is accompanied by an increased uptake of glucose and potassium by the liver. Concurrently, plasma insulin in the portal blood increases significantly. This increase appears to be a result of increased insulin secretion caused by ouabain.

Cyclic 3',5'-adenosine monophosphate and bronchial tone.

The present studies demonstrate that adenylate cyclase and cyclic 3',5'-adenosine monophosphate (cAMP)-phosphodiesterase activities in dog bronchus are comparable to those found in other smooth muscle preparations. Catecholamines, in the order isoproterenol greater than epinephrine greater than norepinephrine, increase the rate of cAMP formation. This effect can be competitively inhibited by propranolol and potentiated by a cAMP-phosphodiesterase inhibitor. The kinetic study of bronchial cAMP-phosphodiesterase showed two different rates of cAMP hydrolysis, with apparent Km values of 1.4 and 48.0 muM. The high affinity cAMP-phosphodiesterase was inhibited competitively by theophylline and papaverine, the latter being about 20 times more potent than the former. The potency of each compound to inhibit the enzyme and to relax the bronchial strip was comparable. These results, the similar order of potency of the catecholamines to relax the bronchus and to increase the rate of cAMP formation, the competitive inhibition of both effects by propranolol, and the relaxing effect of dibutyryl cAMP on bronchial strip, are compatible with the assumption that the cAMP system is one of the biochemical mechanisms mediating bronchial smooth muscle relaxation.

Halothane effect on cAMP generation and hydrolysis in rat brain.

The volatile anesthetic halothane increased the rate of cAMP generation and decreased the rate of cAMP hydrolysis in rat cerebral cortex and cerebellum. The effect of halothane on the enzymes was reflected in a two-fold rise in cAMP content of cerebral cortical tissue exposed to the anesthetic at 3 vol% for 15 and 30 min. The action of halothane on adenylate cyclase is calcium-independent and different from the action of guanine nucleotides, sodium fluoride and specific transmitters. The Vmax of the enzyme is higher in the presence of the anesthetic. It is suggested that halothane, through conformational changes of the enzyme, renders more catalytic sites operative.

Effect of halothane on myocardial cyclic AMP and cyclic GMP content of mice.

Halothane, in anesthetic concentrations (0.6-1.8 volumes/100 ml), produced a dose-dependent decrease in myocardial cyclic AMP (cAMP) content and an increase in cyclic GMP (cGMP) content in mice exposed to a continuous flow of the anesthetic carried in air for 15 min. Atropine (up to 20 mg/kg i.p.) did not alter significantly the myocardial cyclic nucleotides content or the effect of halothane on cAMP and cGMP content. Prazosin and yohimbine had no significant effect on cAMP or cGMP content in the absence of halothane. Both alpha adrenergic antagonists inhibited the halothane-induced increase in cGMP content (ID50, 0.24 and 0.54 mumol/kg i.p. for prazosin and yohimbine, respectively). In contrast, the decrease in cAMP content induced by halothane was not altered by alpha adrenergic antagonists. Propranolol (2 mg/kg i.p.) diminished myocardial cAMP level and prevented the halothane effect on myocardial cAMP content. Pretreatment with 6-hydroxydopamine did not change the cGMP response to halothane. Thus, the action of halothane on myocardial cyclic nucleotides content appears to be predominantly a peripheral effect, not related to cellular mechanisms mediated by muscarinic receptors. The results suggest that the increase in cGMP content induced by halothane does not require intact adrenergic nerve endings and that cellular processes associated with the alpha adrenoceptor system may be involved; the decrease in cAMP content may be due to an inhibition of the beta stimulatory action of catecholamines on adenylate cyclase.

Effect of benzothiadiazine derivatives on cyclic nucleotide phosphodiesterase and on the tension of the aortic strip.

Diazoxide and chlorothiazide (0.1--1.5 mM) had a dose-dependent inhibitory effect on the rate of cAMP and cGMP hydrolysis determined in a 500-g supernatant of rat aorta homogenates; both compounds were weaker inhibitors of cAMP and cGMP hydrolysis than theophylline. cAMP and cGMP content of the aorta did not change in the presence of diazoxide or chlorothiazide; diazoxide, however, further increased the isoproterenol-induced rise in cAMP, while chlorothiazide did not. Both benzothiadiazines decreased the maximum tension of the aortic strip induced by serotonin, phenylephrine or potassium. Diazoxide was a stronger and chlorothiazide a weaker inhibitor of the contractile response than theophylline. Comparison of the biochemical and functional effects of diazoxide and chlorothiazide indicates that the inhibitory effect of these compounds on cyclic nucleotide phosphodiesterase does not by itself explain their vasodilating effect.

Effect of albuterol and terbutaline, synthetic beta adrenergic stimulants, on the cyclic 3',5'-adenosine monophosphate system in smooth muscle.

The effect of albuterol and terbutaline on the cyclic 3',5'-adenosine monophosphate (cAMP) system was studied in rat uterus, aorta and myocardium and in dog bronchus, and was compared to that of isoproterenol in order to determine whether the tissue specificity observed in their functional effects is reflected in their effect on the cAMP system. Tissue specimens were either homogenized in Tris buffer for enzyme activity measurements or incubated in Krebs-Ringer-bicarbonate medium with the test drugs. Both albuterol and terbutaline produce an increase in cAMP content in the tissues due to a direct effect on adenylate cyclase. This effect can be potentiated by a phosphodiesterase inhibitor and antagonized by a beta adrenergic blocking compound. The cAMP response to each beta adrenergic agonist differs in the tissues examined: in uterus and aorta where the maximal effects are idenitcal, the ED50 values may reflect differences in affinity which may account for the different cAMP response to the compounds at the lower concentrations. In bronchus and myocardium, both the maximum effect and ED50 values of the compounds are different. Albuterol and terbutaline increases cAMP content in bronchus significantly and have only a small effect on cAMP cont in myocardium, whereas isoproterenol increases cAMP level significantly in both tissues. The results indicate that the tissue specificity of albuterol and terbutaline may have its origin at the level of the cAMP system.

Plasma retention and metabolic fate of hemoglobin modified with an interdimeric covalent cross link.

Hemoglobin (Hb) modified with an interdimeric bicovalent cross link using 2-nor-2-formylpyridoxal 5'-phosphate (NFPLP) as the cross-linking agent has an exceptionally low O2 affinity (P50 = 47 torr), enabling it to deliver more oxygen at tissue pO2 than blood. In addition, the covalent cross link prevents dissociation of the HbXL tetramers. By using 3H-labeled HbXL, the present study investigated intravascular retention time of cross linked Hb (HbXL), organ distribution, and routes by which HbXL is metabolized and eliminated from the body. The rats were injected with an i.v. bolus (125-200 mg Hb/kg body weight) of either 3H-labeled HbXL or noncross-linked pyridoxal 5'-phosphate modified Hb (diPLPHb) as a control. Urine and feces were collected daily for up to 9 days. Organs and tissues were harvested either at 9 hr or 9 days and assayed for 3H-label content by standard liquid scintillation counting. Plasma retention of HbXL at this dose was about three times longer than diPLPHb, and no HbXL as such was recovered in the urine. HbXL did undergo metabolic degradation in the body, with labeled fragments (mol. wt. less than 10,000) being excreted by the kidneys and gastrointestinal tract. Total body clearance of the label by 9 days amounted to approximately 83% of the injected dose.

Use of metyrosine in the anaesthetic management of patients with catecholamine-secreting tumours. A case report.

Metyrosine 1.5 g daily for days decreased 24-h urine metanephrine concentration by about 60% in a patient with multiple catecholamine-secreting paragangliomas. Despite the considerable inhibition of catecholamine synthesis, this patient exhibited stress-induced sympathetic overactivity, indicated by increases in arterial pressure and serum catecholamine and urine metanephrine concentrations. It is concluded that metyrosine should be introduced early to the preoperative regimen. In this way, optimal inhibitory effect on catecholamine synthesis can be obtained and maintained for a sufficient time to allow catecholamine stores to become as close to normal as possible. Attainment of the optimal therapeutic effect is not clearly defined, but would seem to be best gauged by a combination of clinical tests of sympathetic responses and of suppression of urinary excretion of metanephrines or VMA.

Developmental changes in adenylate cyclase activity in canine myocardium.

Basal and epinephrine-induced adenylate cyclase activity in homogenates of dog myocardium increased significantly from birth to adulthood, without further change with advancing age. The stimulatory effect of epinephrine (i.e. the net increase over basal activity), however, increased (p less than 0.05) during the first week after birth only and then remained constant. While fluoride-stimulated activity increased, 5'-guanylyl imidodiphosphate [Gpp(NH)p]-stimulated activity declined gradually during development (p less than 0.05). In the presence of Gpp(NH)p and epinephrine at concentrations producing a maximal effect, the enzyme activities in all dogs, except in the 1-day-old, were not significantly different and were comparable to that of fluoride-stimulated activity in adult dogs. The results suggest that age-related alterations in the characteristics of the guanine nucleotide regulatory protein may account, at least in part, for the changes in adenylate cyclase activity occurring during development.

Effects of halothane on the cyclic 3',5'-adenosine monophosphate enzyme system in human platelets.

A study of the effects of halothane on the cyclic 3',5'-adenosine monophosphate (cAMP) system in human platelets was undertaken since cAMP has been implicated in the regulation of the process of platelet aggregation and this anesthetic has been reported to decrease platelet aggregation and, in other tissues, to increase adenylate cyclase activity. When exposed to halothane 0.5 to 10 vol%, adenylate cyclase activity was increased in the platelet preparation in a dose-dependent manner, reaching a maximum at 5 vol% (93% increase above basal activity). Platelet aggregation was also inhibited by halothane in a dose-dependent manner, with a maximum effect at about 5 vol% halothane (a decrease of 70%). Kinetic analysis of platelet cAMP-phosphodiesterase suggested two forms of activity, neither of which was altered by halothone. The results that the impairment of platelet aggregation observed with halothane may be brought about by the halothane-induced activation of platelet adenylate cyclase, which may result in a higher cAMP level, inhibiting platelet aggregation.

Primary aldosteronism with uncommon complications.

In a patient who had primary aldosteronism and severe total-body potassium depletion muscular tonic contractures developed during induction of anesthesia. After correction of the potassium deficit, the patient underwent uneventful anesthesia and transabdominal right adrenalectomy. Neither serum potassium level nor EKG seems to provide a reliable index of correction of potassium deficit. Measurement of potassium balance provided a method of quantitating the potassium depletion and of determining when the potassium deficit had been corrected. Balance studies should be utilized preoperatively when long-term potassium loss is suspected to reduced complication secondary to hypokalemia.

Halothane inhibition of canine myocardial adenylate cyclase--modulation by endogenous factors.

We have hypothesized that the halothane-induced depression of myocardial contractility can be explained, at least in part, by halothane's depression of adenylate cyclase, previously demonstrated in whole homogenates of myocardial tissue. Canine myocardial sarcolemmal membranes, which contain the adenylate cyclase of myocardial cells, were separated from other cellular constituents. Halothane did not depress catecholamine-stimulated adenylate cyclase activity in this preparation. Reconstitution of the sarcolemmal membrane preparation with a 100,000 X g adenylate cyclase-free supernatant restored the depressant effect of halothane on adenylate cyclase stimulated by guanosine triphosphate (GTP) 100 microM alone (-55%, P less than 0.01) or in combination with l-isoproterenol 1 microM (-38%, P less than 0.05) or 2.5 microM (-40%, P less than 0.01). Dilution of the supernatant to half-strength decreased the magnitude of the halothane-induced depression of adenylate cyclase activity to 19% (P less than 0.01); at one-quarter dilution, the effect was no longer significant. This study demonstrates the presence of endogenous modulators of the action of halothane on canine myocardial adenylate cyclase that can be reversibly separated from the adenylate cyclase complex.

Effect of enflurane on cerebellar cGMP and on motor activity in the mouse.

The effect of enflurane on the cerebellar content of the intracellular mediator cyclic 3', 5'-guanosine monophosphate (cGMP) and on motor activity was studied in mice. Seizures, as an index of increased motor activity, associated with an increase in cerebellar cGMP content were induced with isoniazide or picrotoxin. Enflurane 0.28-1.68 vol% produced a dose-dependent, reversible decrease in cerebellar cGMP (by about 50% at 0.28 vol%) and delayed or prevented both the increase in cerebellar cGMP and the convulsions induced by isoniazide. Enflurane also protected against picrotoxin-induced convulsions, but not against strychnine-induced convulsions which presumably do not involve cerebellar mechanisms. These results indicate that enflurane affects the cerebellar mechanisms controlling motor activity and it is postulated that this action contributes to the decrease in muscle tone induced by enflurane.

Halothane effect on cGMP and control of motor activity in mouse cerebellum.

The effect of halothane on cerebellar control of motor activity and on cerebellar cyclic 3',5'-guanosine monophosphate (cGMP) content was studied in mice. Isoniazide and picrotoxin were used to increase motor activity and induce seizures associated with an increase in cerebellar cGMP content. Halothane markedly decreased the cerebellar cGMP content (by 60 per cent at 0.61 per cent, the concentration at which 50 per cent of mice lost righting reflex) and prevented the isoniazide-induced increase in cGMP content. Halothane, 0.61 per cent, significantly reduced both isoniazide- and picrotoxin-induced motor activity; the ED50 convulsive dose of isoniazide (137.7 +/- 7.04) and of picrotoxin (1.9 +/- 0.2 mg x kg-1, sc) was about three times higher (402.2 +/- 17.9 and 5.8 +/- 0.6 mg x kg-1, sc, respectively) in mice exposed to halothane. In contrast, halothane did not alter the ED50 convulsive dose of strychnine, which has a different site and mechanism of action, blockade of glycine receptors, a mechanism not involving the cerebellar system. These results indicate that halothane has a significant effect on the cerebellar control of motor activity and that cGMP plays an important role in the alteration of cerebellar function by halothane.