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PubMed1977

Use of calcium ionophores to determine the effects of intracellular calcium on the action potential of canine cardiac Purkinje fibers.

The effects of calcium ionophores X-537A, A23187, and PR-47 on the action potential of canine cardiac Purkinje fibers were studied using standard microelectrode techniques. The ionophores hyperpolarize the membrane potential, eliminate the notch on the action potential plateau, decrease action potential duration, decrease plateau amplitude and duration, decrease spontaneous diastolic depolarization and excitability, and decrease the rate effect on action potential duration. These effects were shown to be independent of catecholamines, membrane-bound calcium, and the patency of the slow inward current channel. The effects of the ionophores depended on the presence of calcium in the external solution. A plausible interpretation of the data is that an ionophore-mediated increase in intracellular calcium shifts the voltage dependence of transmembrane potassium currents to more negative levels.

Action Potentials↗
PubMed1981

Characterization of [3H](+/-)carazolol binding to beta-adrenergic receptors. Application to study of beta-adrenergic receptor subtypes in canine ventricular myocardium and lung.

[3H](+/-)Carazolol, a newly available beta-adrenergic receptor antagonist, can be used to characterize beta-adrenergic receptor subtypes present in membrane vesicles derived from canine ventricular myocardium and canine lung. [3H](+/-)Carazolol binding is saturable, of high affinity, and is displaceable by beta-adrenergic agents in accordance with their known pharmacological potencies. The interaction of carazolol with beta-adrenergic receptors is stereospecific; the (-) stereoisomer demonstrates greater potency than the (+) stereoisomer. Kinetic analysis of [3H](+/-)carazolol interaction with beta-adrenergic receptors suggests that presence of two phases of interaction, consistent with initial rapidly reversible "low" affinity association of ligand with receptor, followed by isomerization to form a high affinity, slowly reversible complex. Through use of a [3H](+/-)carazolol binding assay based on the high affinity complex, pharmacological specificities of beta-adrenergic receptor populations of canine myocardium and lung were quantified. Analysis using computer-assisted techniques suggests a beta 1/beta 2 receptor ratio of approximately 85%/15% for canine myocardium and 5%/95% for canine lung. In the absence of added guanine nucleotides, comparison of potencies of beta-adrenergic agonists in the two membrane systems suggests significant beta 2 selectivity of l-isoproterenol and l-epinephrine, and non-selectivity of norepinephrine. In the presence of saturating levels of guanine nucleotides, comparison of agonist potencies confirms the non-selectivity of l-isoproterenol and l-epinephrine, and beta 1 selectivity of norepinephrine. These results demonstrate that the state of guanine nucleotide regulation of receptors should be defined when examining agonist selectivities for beta-adrenergic receptor subtypes in vitro.

Adrenergic beta-Antagonists↗
PubMed1982

Regulation of large coronary arteries by beta-adrenergic mechanisms in the conscious dog.

We examined, in conscious dogs, the effects of beta-adrenergic stimulation on measurements of left circumflex coronary arterial diameter and blood flow and on calculations of late diastolic coronary resistance (LDCR) and left circumflex coronary internal cross-sectional area (CSA). Isoproterenol (0.1 microgram/kg) initially decreased mean arterial pressure by 25 +/- 2% (mean +/- SEM), and LDCR by 62 +/- 4%, and increased heart rate by 82 +/- 10%, left ventricular (LV) dP/dt by 79 +/- 12%, and mean coronary blood flow by 85 +/- 5%, while CSA rose slightly. The peak effects on CSA (24 +/- 2%) occurred later, along with decreases in mean arterial pressure (7.4 +/- 1.0%) and LDCR (25 +/-5.3%) and increases in coronary blood flow (14 +/- 2%), LV dP/dt (12 +/- 3%), and heart rate (24 +/- 4%). Pirbuterol (1.0 microgram/kg) induced changes that were qualitatively similar to those induced by isoproterenol. Prenalterol (20 micrograms/kg), a cardioselective beta 1-adrenergic receptor agonist, did not affect mean arterial pressure, but increased heart rate by 40 +/- 5%, LV dP/dt by 72 +/- 10%, mean coronary blood flow by 34 +/- 11%, and CSA by 26 +/- 3%, and decreased LDCR by 29 +/- 5+. Isoproterenol and pirbuterol, but not prenalterol, increased coronary sinus O2 content and decreased A-VO2 difference. After beta 1-adrenergic receptor blockade with atenolol (1 mg/kg), prenalterol no longer induced significant effects, whereas isoproterenol and pirbuterol decreased mean arterial pressure similarly to what was observed prior to blockade, but did not increase LV dP/dt, and induced attenuated increases in mean coronary blood flow, CSA, and decreases in LDCR. Thus, in the intact, conscious animal, large coronary arteries are regulated by beta-adrenergic mechanisms. Surprisingly, a major fraction of large coronary arterial dilation appeared to be either directly or indirectly due to beta 1-adrenergic receptor mechanisms, although beta 2-adrenergic effects were also significant.

Animals↗
PubMed1985

Mechanisms of supersensitivity to sympathomimetic amines in the chronically denervated heart of the conscious dog.

Mechanisms of denervation supersensitivity to sympathomimetic amines were studied in conscious animals. Norepinephrine, 0.1 micrograms/kg, increased left ventricular dP/dt significantly more (4208 +/- 828 mm Hg/sec) in dogs with cardiac denervation than in intact dogs (1029 +/- 280 mm Hg/sec), P less than 0.01, whereas responses to isoproterenol were similar in both groups. Denervation supersensitivity to isoproterenol could be demonstrated only after opposing reflex effects were blocked. After ganglionic blockade, norepinephrine still induced 2- to 3-fold greater increases in left ventricular dP/dt and 3- to 7-fold greater increases in heart rate in cardiac-denervated dogs, whereas isoproterenol and prenalterol, not taken up by presynaptic nerve endings, elicited only 40%-50% greater increases in left ventricular dP/dt and heart rate in dogs with cardiac denervation. The density of beta-adrenergic receptors [( 3H]dihydroalprenolol) was elevated (P less than 0.01) in denervated left ventricles (105 +/- 6.9 fmol/mg protein, n = 8) compared to normal left ventricles (70 +/- 6.3 fmol/mg protein, n = 12). This was accompanied by enhanced isoproterenol-mediated adenylate cyclase activity. However, muscarinic cholinergic receptor density, [( 3H]quinuclidinyl benzilate), decreased from control levels of 251 +/- 11 fmol/mg protein (n = 7) to 193 +/- 14 fmol/mg protein (n = 6). Thus, chronic cardiac denervation results in upregulation of the beta-adrenergic receptor and down-regulation of the muscarinic receptor. The increased beta-adrenergic receptor density and adenylate cyclase stimulation correlated well with the amount of denervation supersensitivity to isoproterenol and prenalterol, but accounted for only a minor fraction of denervation supersensitivity to norepinephrine. The major mechanism of denervation supersensitivity to norepinephrine appears to involve lack of the norepinephrine reuptake.

Adenylyl Cyclases↗
PubMed1985

Alpha and beta-blockade and beta-stimulation in Raynaud's syndrome: a double-blind, placebo controlled, single dose study.

We examined in a double blind fashion and placebo controlled the effects of some alpha and beta adrenergic receptor agonists and antagonists on the recovery of finger skin temperature 12 min after finger cooling (5 min waterbath for both hands) in twelve patients with Raynaud's syndrome. A favourable effect was established on phenoxybenzamine 20 mg as compared to placebo. A significant but rather small effect on orciprenaline 10 mg. The beta-agonists prenalterol (10 mg) and terbutaline (5 mg) did not influence the recovery of finger skin temperature. The beneficial effect of phenoxybenzamine 20 mg was not influenced by the addition of beta-agonists (prenalterol 10 mg or terbutaline 5 mg) or a beta-blocker (propranolol 40 mg). The beta-agonists terbutaline and orciprenaline caused a fall in diastolic pressure and an increase in heart rate. These effects presumably were connected with one collapse and three near-collapses. On alpha- and beta-blocker (phenoxybenzamine and propranolol) a decrease in systolic pressure appeared, whereas diastolic pressure did not significantly differ from the placebo value. While physical exercise is considered to exacerbate the hypotensive effect of alpha-blockers, a fall in blood pressure during physical exercise could not be established in our experiments after the addition of propranolol to the alpha-blocker phenoxybenzamine. Our results suggest that an alpha-blocker is a good choice in Raynaud's syndrome, whereas the addition of a beta-blocker may have some advantages.

Adrenergic Agonists↗
PubMed1985

Hemodynamic response to oral prenalterol in dilated decompensated cardiomyopathy as a result of cardiac and vascular effects.

The initial antifailure efficacy of beta-adrenergic agonists is generally lost during prolonged treatment. The reasons are not fully understood. In 11 patients with advanced cardiac decompensation due to dilated cardiomyopathy, prenalterol, a selective beta 1 adrenergic agonist, improved the left ventricular contractility after acute intravenous and during prolonged oral administration. However, after periods of treatment ranging from 2 to 18 weeks, blood pressure and systemic vascular resistance were raised in each patient. These changes resulted in an increase of the left ventricular afterload which was such as to overwhelm the effects of the enhanced contractility, and to extinguish the initial improvement of the cardiac function and of the clinical condition. Stimulation of the presynaptical beta-receptors facilitating norepinephrine release or of the renin secretion by this beta 1 agonist, may be the causes of the systemic vasoconstriction and of the loss of effectiveness in the long run.

Administration, Oral↗
PubMed1983

Beta-adrenoceptor blocker intoxication: epidemiological data. Prenalterol as an alternative in the treatment of cardiac dysfunction.

1 During the three years 1978--1980 the Swedish Poison Information Centre received reports of 184 patients hospitalized due to beta-adrenoceptor blocker overdosage. Of the 35 patients who developed signs of severe cardiac dysfunction (HR less than 50 beats/min, systolic blood pressure less than 80 mm Hg), 23 had ingested propranolol, 10 metoprolol and 2 alprenolol. 2 The mean value of the defined daily doses (DDD) per 1000 inhabitants per day in Sweden during these years were 11.97 for propranolol, 8.02 for alprenolol and 7.74 for metoprolol. The incidence of severe poisoning due to alprenolol overdosage is lower than expected according to DDD. 3 During 1979 19 persons died from overdosage with beta-adrenoceptor blockers in Sweden: 15 due to propranolol (non-selective, lacks intrinsic sympathomimetic activity), 2 to metoprolol (cardioselective, lacks intrinsic sympathomimetic activity). These findings indicate that severe and even fatal poisoning may occur regardless of the type of beta-blocking agent. 4 The usefulness of prenalterol, a cardioselective beta-adrenoceptor partial agonist, in reversing unwanted cardiac effects of beta-adrenoceptor blocking agents is illustrated by two cases of massive propranolol intoxication (maximal plasma concentrations of propranolol 7.2 and 7.8 mumol/l respectively). Prenalterol in high doses (130 and 280 mg/24 h respectively) restored cardiac function.

Adrenergic beta-Antagonists↗
PubMed1983

The systemic beta-adrenergic pathway to renin secretion: relationships with the renal prostaglandin system.

The relationship between renin release evoked by circulating beta-agonists and the renal prostaglandin (PG) system is incompletely defined. Thus, we evaluated systemic and renal hemodynamic responses to iv (0.5 micrograms/kg X min) and intrarenal arterial (0.6 micrograms/kg X min) infusions of the beta1-agonist prenalterol (PNL) in four separate groups of anesthetized dogs. Consecutive iv PNL infusions (n = 6) resulted in a reversible decrease in mean arterial pressure (130 to 117 mm Hg; P less than 0.05) and increases in cardiac output (3.93 to 4.90 liters/min; P less than 0.001), PRA (1.96 to 5.12 ng/ml X h; P less than 0.01), and 6-keto-PGF 1 alpha levels (190 to 482 pg/ml; P less than 0.01). The glomerular filtration rate and renal blood flow were modestly, but not significantly, decreased by the PNL infusions. In a second group of dogs, the infusion of the PG synthesis inhibitor indomethacin (IN; 10 mg/kg, iv; n = 7) before the second PNL infusion blunted PG increases but did not significantly modify the systemic or renal hemodynamic responses to PNL. The magnitude of the PRA and renin secretory rate (RSR) increases post-IN administration was similar to control values, but the absolute levels achieved were not as great as before IN infusion. To assess the role of the renal baroreceptor pathway to renin release after PNL, a suprarenal clamp was used to maintain a constant renal perfusion pressure during PNL infusion in a third group of dogs. In this group (n = 5), both PRA and RSR (from innervated and denervated kidneys) increased after PNL infusion, although IN again diminished the maximum PRA and RSR responses observed. Finally, the unilateral intrarenal arterial infusion of PNL in the last group of dogs did not alter PRA, RSR, or renal hemodynamics. These results demonstrate that renin release elicited by a circulating beta-agonist functions independently of PG synthesis, and that the pathway operates via an extrarenal mechanism.

6-Ketoprostaglandin F1 alpha↗
PubMed1975

Effects of beta-adrenergic receptor blocking agents on blood pressure in conscious hypertensive rats.

The effects of beta-adrenergic receptor blocking agents administered i.v. on the blood pressure in conscious spontaneously hypertensive rats (SHR), renal hypertensive rats (RHR) and normotensive Wistar strain rats (NR) were studied. dl-Propranolol and dl-YB-2, 1 mg/kg i.v., caused a sustained rise in blood pressure in SHR and RHR. The maximum response of each beta-blocking agent after phentolamine, 10 mg/kg i.v., in SHR and RHR was significantly larger than that in NR. The potency ratio for the hypertensive activities of the 1- and d-isomers of propranolol and YB-2 was similar to the ratio of their beta-blocking activities. The pressor effects of the beta-blocking agents after phentolamine were significantly inhibited by adrenalectomy, reserpinization and pretreatment with hexamethonium. The results suggest that the pressor effect of the beta-blocking agents may be due to their beta-blocking activities and the unmasking of alpha-receptor activities of the blood vessels. Furthermore, the greater pressor effect of the agents observed in hypertensive rats is attributed to a greater activity of the sympathetic nervous system in these rats as compared to normotensive rats.

Adrenalectomy↗