Cardiac electrical responses to catecholamines are differentially mediated by beta 2-adrenoceptors in anesthetized dogs

A comparison of bolus versus continuous cardiac output in an experimental model of heart failure

OBJECTIVE

The majority of studies examining cardiac output measurement have been done in physiologically stable models with low thermal background noise. Research comparing continuous cardiac output (CCO) with bolus thermodilution cardiac output (COTD) measures in human and animal models have reported high correlations, negligible bias, but large limits of agreement. The purpose of this project was to compare CCO with COTD measures in an experimental model of heart failure where the cardiac output values were low and the range was narrow.

DESIGN

A one-group experimental design with preintervention control measures and repeated CCO and COTD measures across nine time points.

SETTING

Cardiovascular research laboratory.

SUBJECTS

Thirty dogs.

MEASURES AND MAIN RESULTS

Univariate and multivariate versions of repeated-measures analysis were used to assess the influences of temperature, weight, and stage of the experimental protocol on CCO, COTD, and the differences between them. The two measures CCO and COTD were assessed for agreement by using methods proposed by Bland and Altman. Two hundred and fifty pairs of measurements were obtained during sinus rhythm. The range for COTD measures was 0.5-4.67 L/min and for CCO measures 1.0-5.40 L/min. Of the 250 cardiac outputs estimated by the continuous method, 73.4% of the values were within +/-15% of that estimated by the repetitive, single thermodilution method. The mean bias for the entire protocol was 0.01 (SD = 0.51) with a range of 4.33 L/min.

CONCLUSION

Agreement between the two measures may be the function of biological variability, responses to anesthesia, and technique. Bland and Altman evaluation demonstrated low bias and precision and similar levels of agreement when compared with previous studies in an experimental model where the cardiac output was low and the range was narrow.

Beta 2-adrenergic receptor actions in neonatal and adult rat ventricular myocytes

The physiological function of beta 2-adrenergic receptors in the neonatal and adult heart is incompletely understood, and possible age-dependent differences in beta 2-receptor actions have not been considered. We used isoproterenol (mixed beta 1- and beta 2-receptor agonist) and zinterol (beta 2-selective agonist) to compare beta-receptor subtype actions in neonatal and adult rat ventricular myocytes. When delivered as a bolus at a final concentration of 10(-7) mol/L, both isoproterenol and zinterol increased the amplitude and hastened the kinetics of the calcium and cell-shortening transients in neonatal myocytes. Under identical experimental conditions, isoproterenol increased the amplitude and accelerated the kinetics of the calcium transient and the twitch in adult myocytes, whereas zinterol did not. In the presence of CGP 20712A (beta 1-receptor blocker), a 100-fold higher concentration of zinterol increased the amplitude but prolonged the duration of the twitch in adult myocytes. To probe the mechanism for this age-dependent difference in beta 2-receptor responsiveness, we compared beta-receptor expression and stimulation of cAMP accumulation in neonatal and adult myocytes. beta-Receptor density was 44,339 +/- 5178 sites per cell in neonatal myocytes and 186,346 +/- 13,356 sites per cell in adult myocytes; the relative proportion of beta 2-receptors was comparable in each (16.7 +/- 2.3% and 16.9 +/- 0.9%, respectively). Isoproterenol induced a large increase in cAMP accumulation in neonatal and adult myocytes (20.0 +/- 1.0- and 20.6 +/- 2.6-fold over basal). In contrast, zinterol evoked a substantial increase in cAMP accumulation in neonatal myocytes but only a minor increase in adult myocytes. These studies provide evidence that at low agonist concentrations, beta 2-receptor activation contributes to the positive inotropic response by increasing cAMP and increasing the amplitude and hastening the kinetics of the twitch in neonatal, but not adult, myocytes. Moreover, these results suggest that age-dependent differences in beta 2-receptor coupling to more distal elements in the signaling cascade can influence myocyte beta 2-receptor responsiveness.

Positive chronotropic and inotropic responses to BRL 37344, a beta 3-adrenoceptor agonist in isolated, blood-perfused dog atria

We investigated the chronotropic and inotropic responses to BRL 37344 (a beta 3-adrenoceptor agonist) and isoproterenol in isolated, blood-perfused dog atria. BRL 37344 (0.1-30 nmol) or isoproterenol (0.001-0.3 nmol) increased the sinus rate and contractile force dose dependently. BRL 37344 was 290 times less potent than isoproterenol to increase sinus rate and 140 times less potent to increase atrial force. Both propranolol and bisoprolol similarly inhibited the positive chronotropic and inotropic responses to BRL 37344 and isoproterenol dose dependently. ICI 118,551 (0.1 and 1 nmol) did not significantly affect the positive cardiac responses to BRL 37344 or isoproterenol. Neither imipramine nor tetrodotoxin significantly affected the positive cardiac responses to BRL 37344. These results suggest that the positive chronotropic and inotropic responses to BRL 37344 are mediated mainly by beta 1-adrenoceptors in the dog heart. It is unlikely that beta 3-adrenoceptors, as previously reported in adipose tissue or gastrointestinal smooth muscle, mediate chronotropic and inotropic responses in the normal dog heart.