Herman B. Daniell

Stereoselective delivery and actions of beta receptor antagonists

These studies have revealed that the delivery and actions of beta receptor antagonist drugs are controlled by a cascade of stereoselective processes involving multiple enzymes, transport proteins and receptors. In essence, the free concentration of the pharmacologically active (-)-enantiomer species of these drugs presented to cell surface beta receptors appears to be a function of the stereoselective clearance by hepatic cytochrome P-450 isoenzymes, enantiomer selective binding to alpha 1-acid glycoprotein and albumin and perhaps predominantly by stereoselective sequestration (and release) by the vesicular amine transport protein within adrenergic neurons. Stereoselectivity in the clearance of beta blocking drugs, which can favor either the (+)- or (-)-enantiomer, only appears to be important for the lipophilic drugs which are cleared by hepatic metabolism. Such stereoselectivity is due to differential stereochemical substrate requirements of individual hepatic cytochrome P-450 isoenzymes. Interindividual variations in the stereoselectivity can occur as a result of differences in the amount and expression of cytochrome P-450 isoenzymes due to genetic predisposition or other factors. In the same context, we have observed a significant correlation between the extent and stereoselectivity of binding of beta blocking drugs to plasma proteins. This is another finding which suggests that variability in the expression of individual proteins involved in the beta blocking drug-protein cascade determines the free concentration of the pharmacologically active enantiomer. However, since most observations have been made in young normal subjects, the extent of stereoselectivity in metabolism, binding and other processes is unknown in the general population where steady-state plasma concentrations can vary widely due to multiple biological factors. The observations from neural studies support the concept that adrenergic nerve endings provide a depot for the stereoselective storage and release of the active enantiomer of beta receptor antagonists. The mechanism of this release appears to involve exocytotic secretion of drug that has been stereoselectively accumulated by the neurotransmitter storage vesicles. In terms of this idea, beta receptor antagonists released during nerve stimulation may achieve concentrations of the (-)-enantiomer within the adrenergic synapse greatly in excess of those found in plasma. Such a mechanism could significantly influence both the intensity and duration of beta receptor blockade in the heart, blood vessels, brain and other target tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

Altered levels of PGF in cat spinal cord tissue following traumatic injury.

Previous studies by others indicated that PGs were present in brain, spinal cord, and c.s.f. of several mammalian species. In the present study we compared levels of PGE and PGF by R.I.A. in spinal cord tissue from traumatized cats and cats pretreated with indomethacin prior to trauma to those of baseline and sham operated controls in order to assess for the first time, to our knowledge, whether meaningful changes in levels of PGE and PGF could be detected which might shed new light on the etiology of spinal cord trauma. Levels of PGF (nanograms/gram wet wt) in the cord segment immediately adjacent to the point of trauma were 8.05 +/- 1.50, and 13.13 +/- 1.38 for baseline and sham operated cats respectively. Spinal trauma led to more than a 100% increase in PGF levels to 29.26 +/- 3.58. Although pretreatment with indomethacin 30 min prior to trauma gave the expected blockade of the PGF response to trauma, a measurable level of PGF (2.55 +/- 0.17) was found in the cord after indomethacin. Cord levels of PGF declined after 3 hr in both sham operated and traumatized animals. PGF was maximally stimulated by trauma during the first 3 hr with little effect at 72 hr. Although carefully examined, PGE levels in cat spinal cord appeared to be virtually unaffected by trauma. These findings clearly demonstrate for the first time that traumatic injury to the spinal cord is accompanied by marked increases in PG levels at the site of trauma, and that the observed elevation in PGF in response to trauma can be blocked by indomethacin in vivo. Whether PGF changes are causally related to the etiology of spinal cord trauma, or merely represent a manifestation of PG release as a result of non-specific tissue injury, remains to be seen.

Cardiovascular effects of diazepam and chlordiazepoxide.

In open-chest dog preparations i.v. administration of diazepam and chlordiazepoxide caused increases in coronary blood flow and cardiac output while reducing blood pressure, heart rate, and myocardial contractile force. The decrease in force was independent of the alteration in coronary flow. Arterial--coronary sinus oxygen difference narrowed and myocardial oxygen consumption decreased. These data show that both diazepam and chlordiazepoxide exert an oxygen conserving action along with an increase in myocardial oxygen delivery and suggest that these drugs may be of benefit to the patient with coronary insufficiency by a direct effect on the cardiovascular system as well as through CNS-mediated antianxiety effects.

Limitation of Myocardial Function by Reduced Coronary Blood Flow during Isoproterenol Action

The interpolated decrease in heart force or isometric systolic tension that occurs during isoproterenol stimulation has been examined in terms of changes in coronary flow and myocardial metabolism. In 67 open-chest dogs under pentobarbital anesthesia, determinations were made of lactate, pyruvate, Po2 and Pco2 in arterial and coronary sinus blood; coronary flow was measured with an electromagnetic flow transducer and ventricular force with a strain gauge arch. Although the characteristic, uncomplicated effect of isoproterenol is a marked increase in coronary flow and contractile force, this is briefly interrupted by a sharp decrease in these functions, and the decrease is associated with evidence of anaerobic metabolism. This decrease is concomitant with decreased coronary perfusion pressure and is intensified by sustained infusion of isoproterenol or by lowered oxygen concentrations in the inspired air. The stage of depressed function is counteracted by mechanical maintenance of high aortic pressure or by slow, controlled heart rate. When uncontrolled, tachycardia due to isoproterenol continues without phasic interruption. The intermediate period of depressed function is interpreted in terms of sharply decreased oxygen delivery when both cardiac rate and force are increased. The hemodynamic and metabolic values for these acutely occurring, reversible extremes have been specified.

Myocardial Contractile Force as Influenced by Direct Coronary Surgery

Abstract Myocardial contractile force (MCF) measured by direct application of a strain-gauge arch to the left ventricular surface was determined intraoperatively in 11 patients having saphenous vein graft (SVG) bypass of acute or chronic coronary arterial obstruction. The MCF determinations with the grafts open (control), occluded, and released demonstrated a consistent reduction (average 31%) in contractility during graft occlusion and a prompt return to control levels following graft release without alteration in other aspects of ventricular function. Similar changes were not observed during graft occlusion if the arch was applied to nonviable (scarred) myocardium or to areas outside the region of graft perfusion. The SVG augmentation of blood flow to acute or chronically ischemic but viable myocardium enhances MCF or isometric systolic tension, from which coincident improvement in ventricular function should be anticipated.

Myocardial Sensitivity to Isoproterenol Following Abrupt Propranolol Withdrawal in Conscious Dogs

Summary The sensitivity of the cardiovascular system to isoproterenol following abrupt cessation of propranolol administration was studied in chronically instrumented conscious dogs pretreated orally with propranolol for 14 days. The effects of intravenous isoproterenol on heart rate, blood pressure, and mean velocity of circumferential fiber shortening (Vef) were determined before and during propranolol administration and then at 12 hr intervals following drug withdrawal. During propranolol administration, all dose-response curves to isoproterenol were predictably shifted to the right. Twenty-four hours following propranolol withdrawal, when plasma levels of drug averaged only 7 ng/ml, the chronotropic response to isoproterenol remained significantly inhibited. In contrast, the Vef response recovered rapidly and at 24 hr equaled the pretreatment control response. Furthermore, between 36 and 60 hr, when plasma propranolol levels averaged less than 1 ng/ml, the Vef dose-response curve to isoproterenol was shifted to the left of control prior to returning to the pretreatment level. This leftward displacement of the Vef response curve was not accompanied by a parallel change in the heart rate response or in afterload. Plasma norepinephrine levels were measured throughout the study and were not altered during drug treatment or following withdrawal. Thus, the data indicate that abrupt cessation of propranolol administration is associated with a period of increased myocardial sensitivity to at least the inotropic effects of isoproterenol and that abrupt withdrawal does not result in acute changes in sympathetic nervous system activity.

Inhibitors of prostaglandin synthesis reverse the effects of chronic β-receptor blockade to attenuate adrenergic neurovascular transmission in dogs

The effects of acute and chronic treatment with β-adrenergic receptor blocking drugs on peripheral adrenergic neurovascular transmission were investigated. Experiments were performed using the blood perfused gracilis muscle preparation in control dogs and in animals treated with single or repeated doses of p-receptor antagonists. After 7 days of d, l-propranolol, l-propranolol, or atenolol administration, the arterial pressor response to sympathetic nerve stimulation was significantly reduced in treated dogs (p < 0.05) when compared with controlor d-propranolol-treated animals. In comparison, acute β blockade produced by a single intravenous dose of propranolol had no effect on the pressor response to nerve stimulation. Inhibition of fatty acid-cyclooxygenase activity by indomethacin or piroxicam enhanced the vasoconstrictor response to sympathetic nerve stimulation in chronic d, l-propranolol-, l-propranolol-, and atenololtreated dogs, but had no effect on vascular neurotransmission in control-, chronic d-propranolol-, or acutely d, l-propranolol-treated animals. The vasoconstrictor response to intraarterial phenylephrine was not significantly altered by chronic propranolol treatment, and measurement of norepinephrine overflow during sympathetic nerve stimulation failed to reveal any difference in neurotransmitter release between control- and propranolol-treated dogs. The results indicate that chronic but not acute β-adrenergic receptor blockade alters signaling at the neurovascular synapse to diminish adrenergic transmission. This effect does not appear to result from a change in postsynaptic α1 receptors or from a decrease in norepinephrine release. The data suggest that the stereoz-selective effect of chronic treatment with β-receptor antagonists to attenuate adrenergic neurovascular transmission may result from an enhanced vasomodulator role for eicosanoids or eicosanoid-dependent processes at the adrenergic neurovascular synapse.

Nitroglycerin and propranolol on myocardial O2 consumption during myocardial ischemia

The effects of i.v. and intracoronary (i.c.) nitroglycerin, i.v. propranolol, and the combination of propranolol and nitroglycerin on myocardial oxygen consumption (MVO2) and lactate utilization were studied in situ ischemic working dog hearts. I.v. nitroglycerin reduced MVO2 9% which was associated with the peripheral vasodilatory actions of the drug. I.c. nitroglycerin which had no detectable effects on the peripheral vasculature had no significant effect on MVO2. I.v. propranolol caused an 8% reduction in MVO2 and this action was associated with a negative chronotropic and a slight negative inotropic effect. However, the combination of propranolol and nitroglycerin which was more effective than either drug alone, reduced MVO2 18% thereby indicating that the effects of the two drugs on oxygen consumption are additive. These results support the concept that the reduction of MVO2 seen with nitroglycerin is principally due to peripheral vasodilatory actions of the drug while that seen with propranolol is due to that drugs effect on the heart and the effects of the two agents are complementary.

STUDIES ON THE RELEASE OF PROPRANOLOL FROM ADRENERGIC NEURONS

ABSTRACT In dogs pretreated with propranolol, nerve stimulation or tyramine produced a simultaneous release of propranolol and norepinephrine from the heart. Direct myocardial stimulation with isoproterenol, however, did not release propranolol. Propranolol release was also observed in vitro with cultured sympathetic ganglia exposed to tyramine or veratridine. The results suggest that nerve endings may provide a presynaptic depot for the storage and release of antagonists as well as transmitter.

Evidence that phentolamine acts centrally to increase the contractile force of the heart

Abstract In an attempt to localize the site of phentolamines positive inotropic effect, the sympathetic nervous system was interrupted at the ganglia, entire spinal cord, or at the level of the first cervical vertebrae. In each case phentolamine failed to elicit a positive inotropic effect, and instead decreased heart force and rate. This indicates that the site of action is mediated in a supraspinal area. Preparations utilizing a cross-perfused technique in which the circulation to the recipient animals head was isolated were used to determined the actions of phentolamine on the brain. In the recipient animal heart force increased significantly with only minimal changes in heart rate and aortic blood pressure. This indication of a central site of action was further defined by removing the influences of all tissue anterior to the superior colliculus. This procedure had very little effect of phentolamines cardiovascular action which points to a brain stem site of action.