Altered inotropic responsiveness to isoproterenol of hearts from spontaneously hypertensive rats during development.

The characteristics of the inotropic responses to isoproterenol (Iso) were investigated in isolated hearts from embryonic normotensive (NT) and spontaneously hypertensive (SH) Wistar-Kyoto rats at 14, 16, 18 and 20 days of gestation and from rats 3 and 10 weeks after birth and in ventricular strips prepared from hearts of newborn and 1-week-old rats. With rates of all preparations maintained constant by electrical stimulation, concentration and age dependent increases in contractility in response to Iso, capable of being blocked by propranolol, were observed. In embryonic hearts, irrespective of Iso concentration or embryonic age, similar effects were observed in hearts from NT and SH animals. However, in all postnatal preparations, the effects of Iso on the SH strain were quantitatively different from those on hearts of NT rats of the same age. As early as 12 hr after birth and through 10 weeks of age, most concentrations of Iso induced inotropic effects that were significantly greater in hearts of the SH than the NT strain and the slopes of the linear portions of the concentration-response curves were significantly steeper. These findings indicate that functional beta adrenergic inotropic receptors are present in the embryonic hearts of both strains and undergo changes during development that are strain dependent.

Adrenergic inotropic responsiveness of embryonic chick and rat hearts.

Rates of isolated embryonic chick hearts (ECH) and embryonic rat hearts (ERH) of various ages were maintained constant by field stimulation and the characteristics of their inotropic responses to isoproterenol (Iso) were investigated. Exposure to Iso produced concentration, age, species and calcium dependent increases in contractility that were prevented by propranolol (3x10(-6) M). The slopes of the linear portions of the concentration-response curves increased with age (14-20 days) in the ERH, the maximal slope and inotropic effect being observed in the 18-day-old heart, whereas they decreased with age (3-7 days) in the ECH, the minimal slope but maximal inotropic effect being observed in the 5-day-old heart. At 10(-7) M, Iso produced approximately maximal responses at all ages in the ECH but elicited only minimal responses at all ages in the ERH, approximately ten times this concentration being required to produce maximal responses in the ERH. Reducing the calcium concentration of the bathing medium significantly increased the sensitivities of the hearts of both species to the inotropic effects of Iso. Iso-induced positive inotropic responses were also demonstrated in isolated, driven ventricles from 4-day-old ECH. These results indicate that functional, beta-adrenergic, inotropic receptors are present in the embryonic hearts of both species and undergo changes in responsiveness during embryonic development which are species-dependent.

Norepinephrine- and isoproterenol- induced changes in cardiac contractility and cyclic adenosine 3':5' -monophosphate levels during early development of the embryonic chick.

Changes in contractility and cyclic adenosine 3':5'-monophosphate (cAMP) levels in response to norepinephrine and isoproterenol were monitored in isolated 4-day-old (noninnervated) and 7-day-old (innervated) embryonic hearts to determine whether a relationship between beta adrenergic receptor, adenylate cyclase and altered cardiac function is established at a very early stage in embryonic development before innervation takes place. Norepinephrine and isoproterenol promoted rapid time- and dose-dependent rises in cAMP levels which were greater in the 4-day-old hearts. These increases paralleled observed functional alterations within a specific range of drug concentrations and time. The elevation of cAMP levels and effect on cardiac function produced by isoproterenol (10(-7)M) were blocked by propranolol (10(-6)M). Dissociations between changes in tissue cAMP levels and cardiac function were also uncovered. Maximal increases in contractility were achieved with lower drug concentrations than were required to promote maximal accumulation of cAMP. Relatively high concentrations of norepinephrine or isoproterenol were less effective than lower concentrations in stimulating contractility but were more effective in promoting cAMP accumulation, The results indicate that both cardiac beta receptors adenylate cyclase are present and functionally related early in embryogenesis before sympathetic innervation occurs and that cAMP accumulation is associated with modulation of contractile activity whithin a certain concentration range and time course.

Uptake of and response to norepinephrine by certain tissues of hypertensive rats.

A decrease in the capacity of the sympathetic nervous system of hypertensive rats to take up norepinephrine (NE) has been previously described. In the present study, rats were made hypertensive by one of the following technics: "adrenal regeneration," figure-of-eight knot around one kidney, ingestion of a 10% NaCl solution. After 2 mo, the adrenal regeneration rats were hypertensive, and there was a significant decrease in the capacity of their hearts to take up NE as well as a significant increase in the inotropic response to the amine. Similar results were obtained with hearts and aortic strips of renal hypertensive rats, as well as of rats made hypertensive by drinking a 10% NaCl solution. In renal hypertensive rats, there was no appreciable difference in the time of appearance of hypertension, the decrease in NE uptake by the heart or aorta, or the increase of the inotropic response. Previous work showing a decrease of NE uptake in rats made hypertensive by administration of deoxycorticosterone has been confirmed utilizing three other models of the disease.

Positive inotropic activity of cholera enterotoxin on the embryonic chick heart.

The characteristics of the positive inotropic effect of different preparations of cholera toxin on the isolated 4-day-old embryonic chick heart were investigated. Crude (CCT) as well as partially purified and purified (PCT)preparations of cholera toxin were shown to have positive inotropic activity. Contractility was increased within 1 minute and reached a maximum at approximately 40 minutes after exposure to toxin. Activities of all three toxin preparations were abolished by heating, their effects were prevented by addition of antitoxin, but nort propranolol, before exposure of the heart to toxin. The positive inotropic effect of PCT was not reversed by washing, but was reversed by subsequent addition of antitoxin. The effects of CCT were reversed by insulin. Effects of both CCT and PCT were potentiated by theophylline, and both toxin preparations elevated tissue levels of cyclic adenosine 3',5'-monophosphate. A filtrate of CCT (MW is less than 30,000) exhibited similar cardiostimulant activity but displayed certain differences from the other toxin preparations. The filtrate was heat stable, and its effects were blocked by pretreatment with propranolol but not by antitoxin. The filtrate of CCT, unlike the toxin-containing preparations, did not stimulate intestinal fluid secretion in the dog.

Acetylcholinesterase and responses to acetylcholine in the embryonic chicken heart.

Three and 4 day old embryonic chicken hearts were examined for their responsiveness to acetylcholine and presence of acetylcholinesterase (AChE) to determine the role of the enzyme in the cardiac effects of the transmitter. The effects of acetylcholine on rate and contractility of 3 day old hearts were indistinguishable from those on 4 day old hearts. The effects were readily blocked by atropine at both stages of development. In 3 day old hearts the responses to acetylcholine were not affected by the AChE inhibitor physostigmine but in 4 day old hearts they were considerably potentiated. The effect of acetylcholine on the rates of 4 day old hearts is of short duration (5 min or less). In 3 day old hearts it persists for a much longer time. Thus, the appearance of AChE in the embryonic heart of the chicken does not seem to modify the responsiveness of the cholinergic receptor to the transmitter.

Effect of ouabain on the innervated and noninnervated embryonic chick heart.

Attempts to assess the importance of the role played by endogenous catecholamines in the positive inotropic response to ouabain have produced contradictory results. The sympathetic nervous system is not present in the 4-day-old chicken embryo heart but is fully developed after 7 days of embryonic life. This was confirmed by the fact 4-day-old hearts do not respond to tyramine and cocaine while the usual positive inotropic and chronotropic responses were observed when these drugs were administered to 7-day-old hearts. In spite of this difference the positive inotropic response to ouabain was virtually identical at these two stages of development.