John T. Hamilton

Protective effect of beta adrenoceptor blockade in experimental coronary occlusion in conscious dogs

To assess the effect of beta blockade and membrane stabilization on the outcome of acute coronary artery occlusion, anesthetized dogs were prepared with a ligature at the origin of the circumflex branch of the left coronary artery under direct vision. The dogs were allowed 48 to 72 hours to recover. A total of 154 dogs randomly allocated to 8 treatment groups were given intravenous injections of saline solution, doses of blocking agents found to block isoproterenol-induced tachycardia by 50 or 100 percent (or both) in the study dogs, or equivalent membrane-stabilizing doses of d-propranolol. Of these, 141 dogs met strict criteria for proper ligation. The 2 hour mortality rates were as follows: 72 percent of 25 dogs given 2 to 3 ml of saline solution, 24 percent of 25 dogs given 0.1 mg/kg and 70 percent of 10 given 1 mg/kg of dl-propranolol (Inderal®); 28 percent of 25 dogs given 0.2 mg/kg and 17 percent of 12 given 3.2 mg/kg of MJ-1999 (Sotalol®); 69 percent of 16 dogs given 0.07 mg/kg of dpropranolol; 17 percent of 12 dogs given 0.07 mg/kg of d-propranolol + 0.2 mg/kg of Sotalol; and 31 percent of 16 dogs given 1.4 mg/kg of AY-21,011 (Practolol®). All but 6 deaths were due to ventricular fibrillation. The larger dose of dl-propranolol was associated with more conduction defects, cardiac standstill and greater mortality than the smaller dose. Thus, significant protection (P <0.001) was conferred by the smaller dose of dl-propranolol, both doses of Sotalol, the mixture of d-propranolol and Sotalol, and Practolol. Similar patterns were seen with 24 hour mortality rates. The study shows that protection is due mainly to the beta receptor blocking rather than the membrane-stabilizing actions of these agents.

Relaxations of guinea-pig fundic strip by adenosine, adenine nucleotides and electrical stimulation: Antagonism by theophylline and desensitization to adenosine and its derivatives

Abstract Adenosine and related compounds produced a concentration-dependent relaxation of the guinea-pig fundic strip in non-treated preparations and in preparations treated with atropine, tetrodotoxin (TTX), guanethidine, reserpine or 6-hydroxydopamine (6-OH-DA). The relative potencies in descending order were ATP ⩾ ADP + adenosine. Theophylline (THEO) at low concentrations (25–75 μM) selectively antagonized the inhibitions produced by ATP, ADP and adenosine without antagonizing those due to noradrenaline (NA), isoprenaline (ISO) and papaverine (PAPAV). Schild plots yielded apparent pA2 values for THEO of1.4 × 10−5M, 1.8 × 10−5 M and 2.8 × 10−5 M for ATP, ADP and adenosine respectively, with slopes which were not significantly different from 1 (p + 0.05). These results suggest competitive antagonism. Theophylline was a more specific antagonist of the relaxations induced by the purines than were compounds such as quinidine, phentolamine or 2,2′-pyridylisatogen (PIT). In atropine (1 × 10−7 to 5 × 10−7 M) pretreated fundic strips, transmural or periarterial nerve stimulation produced relaxations which were frequency-dependent. The effects of periarterial nerve stimulation were abolished by phentolamine plus propranolol, by guanethidine, by bretylium, by TTX and by pretreatment with reserpine or 6-OH-DA. The responses to transmural nerve stimulation were abolished only by TTX. The relaxations induced by transmural nerve stimulation were reduced or abolished by THEO, whereas those produced by periarterial nerve stimulation were not affected. Theophylline was more selective than quinidine or phentolamine in blocking such responses. Exposure of the tissues to high concentrations of ATP, ADP or adenosine caused tachyphylaxis to the relaxations induced by the purines as well as those to transmural nerve stimulation, while the responses to NA, ISO, PAPAV or periarterial nerve stimulation were either augmented or unaffected. The effects of desensitization were reversible. This results suggests that the response to transmural nerve stimulation may be due to neural release of adenosine or a related compound. These results are discussed in relation to the release of ATP or related nucleotides from purinergic nerves since the responses to the purines and those to transmural nerve stimulation were reduced or abolished by either THEO or by desensitization with ATP, ADP or adenosine. The possibility is raised that an adenine nucleotide or nucleoside released from nerves might have a physiological function in the “receptive relaxation” of the stomach following stimulation of the pharynx and upper oesophagus by passage of a bolus of food.

EFFECT OF GALLAMINE ON CHOLINERGIC RECEPTORS

SummaryGallamine triethiodide causes tachycardia thought to be due to a specific vagolytic action. Recently, Brown and Crout have shown that gallamine may cause increased inotropic and chronotropic activity due to the release of catecholamines from cardiac sympathetic nerves. As a further analysis of the mechanisms, experiments were performed on pithed rats and spinally transected, braindestroyed catsin vivo pretreated with hexamethonium and/or propranolol to determine the nature of the antiacetylcholine action of gallamine on the heart.A significant antagonism by gallamine of the negative chronotropic action of acetylcholine on the heart of the pithed ratin vivo, pretreated with hexamethonium and/or propranolol, has been demonstrated. Similarly, gallamine was shown to have a significant antiacetylcholine activity on the heart of the spinal cat preparation. The effect of hexamethonium on the cat, however, differed from that on the rat, as in the former it caused a significant decrease in the apparent affinity constant of gallamine. The possible mechanisms whereby this is caused and the clinical implications, should the cat be similar to man in this regard, are discussed.This antiacetylcholine activity of gallamine on the heart (negative chronotropic response to acetylcholine) fulfils many of the requirements for competitive antagonism. Parallel results were obtained with atropine except on the hexametho-nium-treated cat. Thus, if atropine acts on the acetylcholine receptor in a competitive manner, it is likely that gallamine does also.Experiments on the isolated guinea pig ileumin vitro have re-demonstrated the remarkable selectivity of the parasympatholytic activity of gallamine with respect to the cardiovascular system and an interesting antihistaminic action which deserves further study.As the observed anticholinergic actions of gallamine on the heart of both species were obtained with doses necessary for muscle relaxation, it is conceivable that a true atropine-like action may complement any sympathomimetic action of gallamine in producing the tachycardia encountered in clinical practice.RésuméOn croyait que la triéthiodure de gallamine produisait une tachycardie à cause de son action vagolytique spécifique. Récemment, Brown et Crout ont démontré que la gallamine pouvait produire un effet inotropique et chronotropique accru à cause de la libération de cathécolamines par les nerfs sympathiques cardiaques. En guise ďanalyses plus complètes de ces mécanismes, nous avons fait des expériences sur des rats décérébrés et cordotomisés, sur des chats vivants au cerveau détruit et traités au préalable au propranolol et/ou à ľhexaméthonium pour préciser la nature de ľactivité antiacétylcholine de la gallamine sur le cœur. Les courbes dose-réponse de ľactivité chronotropique négative de ľacétylcholine donnée par voie endoveineuse obtenues après la gallamine par voie endoveineuse et/ou ľatropine confirment ľactivité “atropine-like” de la gallamine réalisant plusieurs des exigences comme antagoniste compétitif. Les affmites constantes apparentes pour ľatropine et la gallamine sont différentes chez le rat et chez le chat. Des études faites sur ľiléon du cobaye confirment la sélectivité cardiovasculaire de ľaction antimuscarinique de la gallamine. Ľantagonisme intéressant de la gallamine par ľhexaméthonium sur le coeur du chat est un exemple ďinteraction des médicaments que nous avons discutée. Cette activité antiacétylcholine de la gallamine s’ajoute, selon toute vraisemblance, à toute libération de cathécolamines pour produire la tachycardie observée en clinique.

Propofol induces dilation and inhibits constriction in guinea pig basilar arteries.

The present study investigated the direct action of propofol on guinea pig basilar arterial rings and the possible underlying mechanisms.Arterial rings, with and without endothelium, were mounted in an organ bath and connected to an isometric tension recording system. The effects of propofol (0.63-20 micro gram/mL) were compared with those of Intralipid Registered Trademark (n = 13) after equilibration and precontraction by 5-hydroxytryptamine (5-HT). In another group (n = 8), after pretreatment with either propofol (5 micro gram/mL) or Intralipid Registered Trademark, a contraction by 35 mM KCl was obtained and compared. Another group (n = 12) were incubated in Ca2+-free Krebs buffer and after depolarization by 45 mM KCl, a dose-response curve to CaCl2 was obtained to compare the effect of propofol (5 micro gram/mL) and Intralipid Registered Trademark on the influx of Ca2+. Finally, in Ca2+-free Krebs buffer, the effect of Intralipid Registered Trademark or propofol on 5-HT-evoked contractions (n = 6) were assessed. Propofol caused significant dilation with or without endothelium present, inhibited KCl-induced contraction, and significantly lowered the dose-response curve for CaCl2. In Ca2+-free Krebs buffer, propofol significantly inhibited 5-HT-evoked contraction, which is dependent on intracellular Ca2+-release. In conclusion, propofol inhibited vasoconstriction and induced vasodilation by mechanisms consistent with reduced extracellular calcium influx and suppressed intracellular calcium release. (Anesth Analg 1996;83:472-6)

Antagonism by methylxanthines of purine nucleotide- and dipyridamole-induced inhibition of peristaltic activity of the guinea pig ileum

Abstract The effect of the methylxanthines (aminophylline, AMI and theophylline, THEO) on the responses to the purine pig ileum has been studied. ATP, ADP and adenosine produced concentration-dependent inhibition of peristaltic activity in non-treated, reserpine- or 6-hydroxydopamine-pretreated preparations; the relative potencies in descending order were ATP ⩾ ADP > adenosine. This inhibition of peristaltic activity by the purine nucleotides was antagonized by low concentrations of the methylxanthines and this effect was reversible at all concentrations tested. The antagonism by THEO (5.0 μM) appeared competitive in nature. In contrast, peristaltic acdtivity inhibited by noradrenaline, isoprenaline, periarterial nerve stimulation (PNS) or papaverine was unaffected by the presence of the methylxanthines. Dipyridamole (5–10 μM) produced and inhibition in the late phase of the peristaltic activity, while lower concentrations (0.5–1.3 μM) potentiated the inhibitory response to ATP, ADP and adenosine without influencing the responses to noradrenaline, isoprenaline, PNS or papaverine. The methylxanthines selectively antagonized the inhivitory effects of dipyridamole alone or in combination with the purine nucleotides. It is proposed, that AMI and THEO exert a selective action on a specific ATP-receptor and that THEO may be a competitive antagonist of the purine nucleotides on peristaltic activity of the guinea pig ileum. Since 100-fold greater concentrations of AMI ot THEO are required to inhibit this preparation by inhibition of phosphodiesterase, it is highly unlikely that the inhibition of peristalsis by dipyridamole and the subsequent blockade by low concentrations of the methylxanthines involves the 3′, 5′-cyclic AMP pathway. Furthermore, the parallelism observed, between the effect of the methylxanthines on the inhibitory responses obtained with dipyridamole nad the effect of ATP, ADP and adenosine, supports the hypothesis that non-adrenergic, non-cholinergic inhibitory neurons releasing a purine nucleaotide as the neurotransmitter contribute to the descending inhibition observed during peristalsis.

Propofol differentially attenuates the responses to exogenous and endogenous norepinephrine in the isolated rat femoral artery in vitro

Propofol causes a decrease in vascular resistance mediated in part by a decrease in sympathetic output.To determine whether attenuation of norepinephrine release from sympathetic perivascular nerve terminals could contribute to decreased vascular resistance, we examined the effects of propofol on the contractile responses to exogenous and endogenous norepinephrine in the rat femoral artery. Endogenous norepinephrine was released from sympathetic nerve terminals using electrical field stimulation. The responses to both exogenous norepinephrine and neurally released norepinephrine were attenuated by propofol in concentrations from 1.0 to 10.0 micro gram/mL. At 50% of maximal and at maximal contractile responses to norepinephrine and electrical field stimulation, the response to electrical field stimulation was inhibited to a greater extent than the response to exogenous norepinephrine. This suggests that, in addition to direct postsynaptic vasodilation, propofol has the presynaptic effect of inhibiting norepinephrine release from perivascular nerves. (Anesth Analg 1995;80:793-9)

An open label trial of the possible analgesic effects of dipyridamole

There is experimental evidence that adenosine and the adenosinergic agent dipyridamole may inhibit the activity of nociceptive neurons. Dipyridamole was given to 15 patients with chronic pain in an open uncontrolled investigation. Eleven patients tolerated the drug well and seven of these patients obtained subjective benefit. The results are held to justify further investigation under controlled blind conditions. A possible ceiling effect of dipyridamole was noted.

Conditions affecting the interaction of cholinergic agents on the longitudinal muscle of the guinea‐pig ileum—unexpected effects with hexamethonium

Affinity constants were determined for atropine and N‐methyl atropine, using both acetylbetamethylcholine and carbaminoylcholine as agonists, on plexus‐containing and plexus‐free preparations of the guinea‐pig ileum. Hexamethonium (1 times 10−4M) decreased the apparent affinity constant of atropine on the plexus‐free preparation when carbaminoylcholine but not acetylbetamethylcholine was the agonist. With the latter agonist both antiacetylcholine drugs were equiactive. Hexamethoniums selective action was associated with a significant shift to the left of the log dose‐response curve to carbaminoylcholine on the plexus‐free preparation. While others have observed that hexamethonium may be atropine‐like, samples of hexamethonium have been reported as being contaminated with a “depolarizing compound”, which may account for the results presented in this study. Nevertheless, the present results are consistent with an allosteric rather than a direct binding mechanism in the blocking action of atropine. The observation that the agonist, acetylbetamethylcholine, was not similarly affected by hexamethonium (or any possible contaminant) suggests that agents with primarily muscarinic activity may not be so affected. This study is another example emphasizing the care needed in interpreting data obtained with the aid of “pharmacological tools”.

Ineffectiveness of beta-adrenergic blockers on ventilatory response to carbon dioxide

The effects of the beta‐adrenoceptor blockers atenolol, metoprolol, pindolol, and propranolol on the ventilatory response to carbon dioxide rebreathing have been determined in a double‐blind randomized manner. Eight healthy, male, nonsmoking subjects received cumulative doses of each drug over a 10‐hr period. The effects of each drug on heart rate and carbon dioxide sensitivity were determined at intervals of 2 hr and were related to plasma concentrations of each drug. Maximum reduction of exercise heart rate was achieved with all four beta blockers and plasma concentrations were in the usual therapeutic range for these drugs. There was considerable intersubject and within‐subject variability in ventilatory responsiveness to inhaled carbon dioxide, but we were not able to discern any alteration in central sensitivity to increasing carbon dioxide concentrations.

Ketamine and the neuromuscular junction. Interpretation of the mechanism of some clinical peripheral side effects

Abstract 1. 1. Ketamine has been shown to potentiate and then block, in a dose-related manner, the response both to indirect (nerve) and direct (muscle) stimulation of the rat phrenic-nerve diaphragm in vitro . 2. 2. The action on the response to muscle stimulation was still produced in the curarized preparation. 3. 3. The effects were obtained with both concentrations of ketamine that were less than the concentrations used clinically by intramuscular injections, it is concluded that similar unobserved actions could be occurring in clinical practice. 4. 4. The possible mechanisms and interactions are discussed to expand on an earlier report which needs to be reinterpreted.