Influence of myocardial mechanical activity and coronary blood flow on myocardial digoxin uptake

Relationship between plasma, atrial and ventricular perhexiline concentrations in humans: insights into factors affecting myocardial uptake

AIM

Little is known regarding the steady-state uptake of drugs into the human myocardium. Perhexiline is a prophylactic anti-anginal drug which is increasingly also used in the treatment of heart failure and hypertrophic cardiomyopathy. We explored the relationship between plasma perhexiline concentrations and its uptake into the myocardium.

METHODS

Blood, right atrium ± left ventricle biopsies were obtained from patients treated with perhexiline for a median of 8.5 days before undergoing coronary surgery in the perhexiline arm of a randomized controlled trial. Perhexiline concentrations in plasma and heart tissue were determined by HPLC.

RESULTS

Atrial biopsies were obtained from 94 patients and ventricular biopsies from 28 patients. The median plasma perhexiline concentration was within the therapeutic range at 0.24 mg l⁻¹ (IQR 0.12-0.44), the median atrial concentration was 6.02  mg kg⁻¹ (IQR 2.70-9.06) and median ventricular concentration was 10.0 mg kg⁻¹ (IQR 5.76-13.1). Atrial (r² = 0.76) and ventricular (r² = 0.73) perhexiline concentrations were closely and directly correlated with plasma concentrations (both P < 0.001). The median atrial : plasma ratio was 21.5 (IQR 18.1-27.1), ventricular : plasma ratio was 34.9 (IQR 24.5-55.2) and ventricular : atrial ratio was 1.67 (IQR 1.39-2.22). Using multiple regression, the best model for predicting steady-state atrial concentration included plasma perhexiline, heart rate and age (r² = 0.83). Ventricular concentrations were directly correlated with plasma perhexiline concentration and length of therapy (r² = 0.84).

CONCLUSIONS

This study demonstrates that plasma perhexiline concentrations are predictive of myocardial drug concentrations, a major determinant of drug effect. However, net myocardial perhexiline uptake is significantly modulated by patient age, potentially via alteration of myocardial:extracardiac drug uptake.

Tachycardia alone fails to change the myocardial pharmacokinetics and dynamics of lidocaine, thiopental, and verapamil after intravenous bolus administration in sheep

Previous reports have suggested that tachycardia alone can increase the rate of myocardial uptake of some drugs. As part of a systematic study of the determinants of the myocardial uptake and effects of drugs in critical illness, the effect of tachycardia induced by intracardiac pacing on the myocardial disposition and effects of lidocaine, verapamil, and thiopental were studied in chronically instrumented sheep. For each drug, seven sheep received either 100 mg of lidocaine, 10 mg of verapamil, or 750 mg of thiopental over 2 min in unpaced and paced (140 beats/min) states on separate occasions and in random order. Arterial and coronary sinus (effluent from the heart) blood samples were taken at regular intervals for 30 min, and the maximum rate of change of left ventricular pressure (LV dP/dtmax) was measured as an index of myocardial contractility. There were no differences between unpaced and paced studies in the time courses of arterial and coronary sinus concentrations, or the time-courses of myocardial contractility and blood flow, after bolus iv injections of these drugs. Tachycardia alone does not appear to influence the myocardial kinetics or dynamics of lipophilic drugs that can rapidly diffuse into the heart.

Influence of perfusion flow rate on uptake and pharmacodynamics of quinidine in isolated perfused rat heart

With the single-pass isolated perfused rat heart preparation, we examined the effect of perfusate flow rate on the time course of quinidine output concentration (Cout) and the change in the QT interval (delta QT) on the electrocardiogram. Four hearts were perfused at 5.6 mL/min with quinidine (20 microM) for 25-45 min. This was followed by a 50-min washout period with drug-free perfusate. This procedure was repeated four times in each heart. Using a one-compartment model, the rate constants calculated from the time course of Cout (k) and delta QT (Ke) were similar (p > 0.05) and did not vary among the five phases. A zero-time intercept for Cout indicated shunting of perfusate (1.2-13.6%), from which the coronary output concentration (Ccor) and coronary flow rate were calculated. These experiments were repeated in another six hearts, except flow rates of 2, 4, 6, 8, and 2 mL/min were used in each phase. Equilibration of Cout and delta QT was faster with increasing flow rate, and ke and k were similar at each flow rate (p > 0.05). A modified Kety-Renkin-Crone equation was fitted to values for k and coronary flow rate. The mean permeability surface product estimate was 15.3 +/- 3.0 mL/min/g heart, which in comparison with the highest flow rate used (9 mL/min/g heart) suggests that quinidine has intermediate permeability. The excellent fit obtained indicates that a more complex model, incorporating heterogeneous flows or opening of capillaries at higher flows, was not necessary.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of salbutamol on digoxin pharmacokinetics

A single dose of the beta 2-adrenoceptor agonist salbutamol has previously been shown to decrease serum digoxin concentration in healthy volunteers. A possible explanation of the phenomenon is a beta 2-adrenoceptor-mediated increase in the specific binding of digoxin to skeletal muscle. The present study was undertaken to further elucidate the effect of salbutamol on the pharmacokinetics of digoxin in man. Nine volunteers were studied on two occasions during salbutamol or placebo treatment. On test days salbutamol, 4 micrograms.kg-1.h-1 or saline was infused for 10 h, preceded and followed by four and three days, respectively, of oral administration. A single i.v. injection of digoxin 15 micrograms.kg-1, was given 20 min after starting the infusion. At the end of the infusion a muscle biopsy was taken from the vastus lateralis. Blood samples for the analysis of serum digoxin and potassium were repeatedly taken over 72 h. Urine was collected over a period of 24 h for determination of the renal excretion of digoxin and potassium. The serum digoxin concentration, expressed as the AUC 0-6 h was 15% lower during salbutamol infusion than during saline infusion. Salbutamol caused significantly faster elimination of digoxin from the central volume of distribution to deeper compartments. Salbutamol had no effect on the renal clearance of digoxin. The skeletal muscle digoxin concentration tended to be higher (48%) during salbutamol compared to placebo treatment. The serum potassium concentration was significantly lower after salbutamol compared to placebo, as was the rate of renal excretion of potassium.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of physical exercise on serum digoxin concentration and heart rate in patients with atrial fibrillation

Heart rate and serum digoxin concentration in eight patients with atrial fibrillation were studied at rest and during exercise when initial serum digoxin concentrations were zero and at low and high therapeutic values. Eight patients with ischemic heart disease and in sinus rhythm were studied for comparison. Though the serum digoxin concentration decreased significantly during exercise, the absolute reduction in heart rate was the same at rest and during exercise in patients with atrial fibrillation. Compared with the control patients in sinus rhythm, the heart rate in patients with atrial fibrillation was not adequately controlled during exercise by any serum digoxin concentration tested despite a reduction in heart rate with increasing digoxin concentration. The effects of digoxin on heart rate regulation in atrial fibrillation are complex and include direct effects on the myocardium as well as indirect effects mediated by modulation of the autonomic nervous system; the present results indicate that the drug is not displaced from the target organs by decreasing serum concentrations during exercise. In atrial fibrillation, because the demands on the filter function of the atrioventricular node are highly unphysiological, the effect of digoxin on heart rate during exercise is not adequate.

Regional pharmacokinetics. I. Physiological and physicochemical basis

Systemic pharmacokinetics is the study of the time-course of drug concentrations in 'systemic' blood sampled from either an arterial, central venous or peripheral venous blood vessel. It is generally not suitable for studying the pharmacokinetics of drugs in individual organs of the body, when the blood concentrations of a drug are changing rapidly, or when the physiological or pathophysiological status of a patient is unstable. Regional pharmacokinetics addresses some of these limitations and is based on the study of the factors influencing drug concentrations in specific regions (tissues, organs) of the body due to the movement of drug from blood into and out of the region (drug 'uptake' and 'elution', respectively). It provides a vital link between systemic pharmacokinetics and molecular pharmacology. The physiological basis of regional pharmacokinetics is a function of the interactions of the drug between the cells and proteins in blood, the blood flow supplying a region, the structure of the capillaries of the region and the types of specific and non-specific binding within the region. The physicochemical basis of regional pharmacokinetics is a function of the factors influencing the rate and extent of diffusion of a drug through aqueous and lipid mediums, such as molecular weight, ionization, charge, and lipophilicity.

Acute myocardial uptake of digoxin in humans: correlation with hemodynamic and electrocardiographic effects

Acute myocardial uptake of digoxin was measured at a constant paced heart rate (75 beats/min) for 30 min after an intravenous bolus injection of 500 micrograms of digoxin in 14 patients with ischemic heart disease. Myocardial digoxin content, determined by serial measurement of aortocoronary sinus digoxin concentration gradients and coronary sinus blood flow, was expressed relative to coronary sinus blood flow at rest and correlated with simultaneous hemodynamic and electrocardiographic changes. Myocardial digoxin uptake was extensive (4.1 +/- 0.7% of total injected dose at 30 min) and prolonged, with rapid initial uptake (75.3 +/- 6.6% of maximum at 3 min), followed by a variable phase of slower accumulation. Peak left ventricular positive first derivative of left ventricular pressure (dP/dt) increased progressively (p less than 0.01), with a similar time course to that of myocardial digoxin accumulation; maximal change was 18.5 +/- 4.7% at 27 min. The ratio of inotropic effect to myocardial digoxin content did not vary significantly over the period of the experiment. However, peak inotropic effects in individual patients were not significantly related to peak myocardial digoxin content. The spontaneous PR interval increased transiently, with a peak increase of 5.9 +/- 1.8% (p less than 0.05) 12 min after digoxin administration. It is concluded that after intravenous bolus administration, 1) peak effects of digoxin on atrioventricular (AV) conduction occur early, whereas positive inotropic effects increase progressively for greater than or equal to 27 min; and 2) digoxin accumulation in the human myocardium is prolonged and is a determinant of inotropic effects, but not of prolongation of AV node conduction.

Effect of salbutamol on digoxin concentration in serum and skeletal muscle

Beta 2-receptor stimulation has been reported to increase the concentration of 3H-ouabain in rat skeletal muscle. The present study was undertaken to see if beta 2-adrenoceptor stimulation by i.v. injection of salbutamol had any influence on the pharmacokinetics of digoxin in man. Ten volunteers were digitalized with digoxin and were investigated on two occasions. On each occasion a muscle biopsy was taken from the quadriceps after 2 h of supine rest. An injection of salbutamol 4 micrograms.kg-1 b.wt. or saline was then given intravenously. Blood samples were taken before and after the injection and further muscle biopsy was taken from the same thigh 120 min after the injection. Compared to the injection of saline, salbutamol caused a decrease in the serum digoxin and potassium concentrations. The change in serum potassium was significantly correlated with that in digoxin. The digoxin concentration in skeletal muscle was not significantly changed by either the salbutamol or saline injections.

Short-term myocardial uptake of lidocaine and mexiletine in patients with ischemic heart disease

Determination of short-term myocardial drug uptake and subsequent redistribution was performed in 27 patients with ischemic heart disease for the antiarrhythmic agents lidocaine and mexiletine, using frequent simultaneous measurements of drug concentration in aortic and coronary sinus blood, combined with measurement of coronary sinus blood flow after intravenous bolus injection of the drug. Maximal myocardial drug content per unit resting coronary sinus blood flow (MDC:F) was significantly greater in patients in whom coronary sinus pacing at 100 beat/min was performed during the initial period of drug uptake. Maximal myocardial drug content occurred after 2.4 +/- 0.2 (SEM) for lidocaine and after 5.5 +/- 0.6 min for mexiletine (p less than .001), and pacing did not affect time to maximum myocardial drug content. In nonpaced, but not paced, patients maximal MDC:F was greater in the lidocaine group than that in the mexiletine group. The subsequent efflux of lidocaine from the myocardium was more rapid that that of mexiletine in both paced and nonpaced groups.

Beta-blockade and binding of digoxin to skeletal muscle

The effect of beta-blockade and a 1-h bicycle exercise test on the digoxin concentration in skeletal muscle (thigh) and serum was studied in 10 healthy men, who had ingested 0.5 mg digoxin daily for 2 weeks. Each subject performed two exercise tests at 100-140 W during maintenance digoxin treatment and 24 h after the latest dose. They rested in the supine position for 2.5 h before the exercise. Sixty minutes before the start of the exercise 0.25 mg/kg b.w. propranolol or saline (control) were injected (single-blind). At the end of the exercise the mean heart rate was 30% lower with beta-blockade (P less than 0.001). During exercise the mean skeletal muscle digoxin concentration increased by 29% (P less than 0.01) in the control situation and by 12% (NS) with beta-blockade. The results indicate that propranolol partly inhibits the exercise-induced increase in skeletal muscle digoxin binding. This might be due to inhibition of a catecholamine-induced stimulation of Na+-K+ATPase during exercise.

Digitalis: neurally mediated arrhythmogenic and coronary vasoconstrictor properties

Studies of digitalis have revealed an appreciable role of the autonomic nervous system in the modulation of the cardiovascular toxicity of digitalis. Neuraxis transection experiments and studies using digitalis compounds that fail to enter the central nervous system suggest that centers located in the floor of the fourth cerebral ventricle play a substantial part in the neural arrhythmogenic effects of the digitalis glycosides. Electrical stimulation of the area postrema in partially digitalized animals, not otherwise manifesting ventricular arrhythmias, elicits ventricular tachycardia. Neuraxis transection experiments and studies with a charged digitalis derivative also suggest localization of the coronary vasoconstrictive properties of digitalis to the area postrema. The results suggest that this region of the brainstem is sensitive to a neurally active substance, digitalis, causing peripheral sympathetic effects of considerable consequence.

Physical exercise and binding of digoxin to skeletal muscle--effect of muscle activation frequency

Ten healthy subjects who had ingested 0.5 mg digoxin daily for at least 10 days, performed a 1-hour bicycle exercise test on two occasions, 24 h after the latest dose, with the same work load but at two different pedalling rates, 40 and 80 rpm. During exercise the mean digoxin concentration in the thigh muscle increased by 8% at 40 rpm (n.s.) and by 29% at 80 rpm (p less than 0.01). The serum digoxin concentration decreased by 39% at both pedalling rates (p less than 0.001). The results suggest that the increase in skeletal muscle digoxin concentration during exercise is related to the neuromuscular activation frequency. The digoxin concentration in erythrocytes was measured in 16 healthy subjects before and 1 minute after a 1-hour bicycle exercise test. The erythrocyte digoxin concentration decreased by 12% (p less than 0.01) during the exercise indicating that the increased uptake of digoxin in skeletal muscle during exercise influences the digoxin concentration in other tissues.

Biochemical regulators in cardiac hypertrophy

In recent years research has shown that muscle is capable of reacting to mechanical stimuli by altering biochemical processes. Myocardium is probably the source of a biochemical factor, or factors which activate myocardial protein synthesis. In experimentally induced cardiac hypertrophy adaptive alterations have been shown to occur not only in the adrenal medulla but also in the adrenal cortex. Finally, detection of cross reactivity between digitalis glycosides and a number of steroid hormones has succeeded. We assume that such cross reactivity indicates the existence of an endogenic factor of steroid character, which is produced in the adrenal gland and functions as an endogenic cardiotonic agent. During experimental cardiac hypertrophy its synthesis is possibly increased. We propose the term "endocardin" or "endocardiotonin" for this agent.

Physical exercise and digoxin binding to skeletal muscle: relation to exercise intensity

The effect of a 1 h bicycle exercise test on digoxin concentration in skeletal muscle (thigh) and serum was studied in 10 healthy men, who had ingested digoxin 0.5 mg daily for 2 weeks. During maintenance digoxin treatment each subject performed 2 exercise tests, at 70-90 W and 140-180 W both 24 h after the last dose, at a 2-7 day interval. During exercise at the lower work load the mean skeletal muscle digoxin concentration increased by 9% (n.s.) and the mean serum digoxin concentration decreased by 26% (p less than 0.001). The high work load induced a mean increase in skeletal muscle digoxin of 20% (p less than 0.05) and a mean decrease in serum digoxin of 40% (p less than 0.001). The results indicate that the increased uptake of digoxin into exercised skeletal muscle and the decrease in serum digoxin during exercise is related to the intensity of the exercise.

The influence of heart rate on digoxin-induced inhibition of myocardial Na+-K+-ATPase activity in the dog

1. Groups of sedated dogs were studied at spontaneous heart rates (HR), 55-100/min, or at paced HR 200/min, with or without intravenous digoxin administration. After 60 min, active rubidium uptake (86 Rb+) of ventricular samples was determined in vitro. 2. Untreated fast and slow HR groups had similar uptakes. Following digoxin, 0.08 mg/kg, uptake was less at fast than slow HR (63.8, s.e.m. = 4.5 v. 87.5, s.e.m. = 5.0 pmol/mg LV/15 min, P less than 0.01). After 0.125 mg/kg, values were again lower in the fast HR group in which five of seven developed ventricular tachycardia. 3. Heart rate does not alter in vitro activity of myocardial Na+-K+-ATPase but does influence inhibition of the enzyme resulting from digoxin administration.

Digoxin concentration in right atrial myocardium, skeletal muscle and serum in man: influence of atrial rhythm

Serum, right atrial myocardium and skeletal muscle collected from 32 adult patients undergoing open heart surgery were analyzed for digoxin by radioimmunoassay. Preoperatively 20 patients were in sinus rhythm, but not in patients with atrial fibrillation, there was a highly significant correlation between digoxin concentration in serum and right atrial myocardium, in skeletal muscle and right atrial myocardium, and in serum and skeletal muscle. The means and variances of the ratios right atrial myocardium/serum and right atrial myocardium skeletal muscle were significantly higher in patients with atrial fibrillation than in those with sinus rhythm. This, plus the lack of difference in ratios skeletal muscle serum between these groups of patients, indicate increased right atrial digoxin binding in atrial fibrillation in man. This conclusion is further supported by the finding of similar or higher digoxin concentration in right atrial myocardium than in left ventricular myocardium in atrial fibrillation (6 patients), and a lower digoxin concentration in right atrial myocardium than in left ventricular myocardium in sinus rhythm (3 patients).

Myocardial ouabain content and susceptibility to ouabain cardiotoxicity associated with circulatory volume overload in the dog

The influence of circulatory volume overload on the myocardial uptake of ouabain and on cardiotoxicity was studied in the unanaesthetised dog with aorto-caval fistula. One hour after tritiated ouabain (0-02 mg/kg IV) both ventricles and atria contained more ouabain than did those of normal dogs (left ventricle (LV), 166+/-23 (SD) ng/g vs. 97+/-19 ng/g, P less than 0-001) while concentrations in skeletal muscle, liver, kidney and plasma were not different in the two groups. In other experiments ouabain was infused to cardiotoxicity (7-5 microgram/kg followed by 3 microgram/kg/min). Cardiotoxicity occurred earlier in dogs with fistula than in normals (16-5+/-2-7 min vs. 24-1+/-2-4 min, P less than 0-001). Ouabain concentrations in myocardium were not different (LV, 434+/-58 ng/g, vs. 442+/-42 ng/g) while concentrations in liver and kidney were less in those with fistula (181+/-35 ng/g vs. 278+/-69 ng/g, P less than 0-001; 1422+/-189 ng/g vs. 2747+/-479 ng/g, P less than 0-001). Average content of skeletal muscle was also less, in proportion to administered dose. The increment in myicardial ouabain content associated with aorto-caval fistula appears to be physiologically active and hence is presumably specifically bound to the digitalis receptor. The observations in this model suggest the possibility of augmented cardiac glycoside uptake in some clinical cardiac diseases.