Effect of muscular exercise on adrenaline and noradrenaline secretion of the adrenal gland in the dog

Cardiac sensitization: methodology and interpretation in risk assessment

An increased sensitivity of the heart to endogenous epinephrine (adrenaline), a phenomenon referred to as cardiac sensitization, has long been recognized as a risk during exposure to hydrocarbons, principally halogenated hydrocarbons. Cardiac sensitization, which can result in serious arrhythmia and death, requires a certain critical blood level of both the sensitizing agent and epinephrine. The original research and methods utilized an exogenous epinephrine challenge during inhalation exposure to a chemical to assess cardiac sensitization potential in Beagle dogs. These screening tests were developed about 30 years ago, although in the last 15 years some modifications of these methods have occurred in response to testing chlorofluorocarbon (CFC) replacements. Results from these experimental cardiac sensitization studies have been used for semi-quantitative risk evaluation for occupational exposures but now are being used more quantitatively for regulatory purposes. The risks associated with cardiac sensitization from CFC replacements are unknown but expected to be low based on cardiac sensitization studies in the 1970s where dogs were made to generate their own adrenaline. With the advent of physiologically based pharmacokinetic (PBPK) modeling, greater emphasis is being placed on quantitative risk assessment for cardiac sensitization. In this investigation, we have examined the various methodologies used for detection of cardiac sensitization and discussed their limitations and advantages. In addition, we examined the potential concerns involved in using experimental cardiac sensitization data and PBPK modeling to predict exposure scenarios.

Distribution and function of beta-adrenoceptors in different chambers of the canine heart

1 An improved binding assay involving (-)-[(3)H]-dihydroalprenolol (DHA) and KCl-washed cardiac membranes was developed to study beta-adrenoceptors in the canine heart quantitatively.2 Receptor numbers varied from 3.8 to 7.1 pmol/g fresh tissue, showing a steady increase from left atrium --> right atrium --> right ventricle --> interventricular septum --> left ventricle. With one minor exception, the same pattern was found for adenylate cyclase activity and Na(+), K(+)-activated ATPase activity.3 The binding of DHA was inhibited in the expected manner by beta-adrenoceptor agonists and antagonists, and was stereospecific, in confirmation of previous studies. Dissociation constants determined from Scatchard analyses included DHA: 2.5 nM; (-)adrenaline: 230 nM; (-)noradrenaline: 1167 nM. Kinetic analyses of the binding of DHA yielded a K(D) of about 4 nM.4 The distribution of beta-receptors is closely related to that of blood flow and the arrival plus retention of a circulating catecholamine, but is markedly different from that of endogenous noradrenaline, and thus adrenergic nerve terminals. Most receptors thus appear not at synapses but diffusely localized where they can react with circulating adrenaline.5 Evidence is discussed that beta-receptors at synapses respond primarily to neural noradrenaline, less to circulating adrenaline, and hardly at all to circulating noradrenaline; responses mediate increased cardiac output during exercise. In contrast most cardiac beta-receptors appear to respond only to adrenaline, and to be used, except at times of severe circulatory stress, during psychological stress.