Alpha 1- and beta-adrenergic and muscarinic-cholinergic regulation in the spontaneous beating and Ca2+ oscillations in cultured neonatal rat cardiac myocytes

Comparative Analysis of Cardiac Effects of α1A-Adrenoreceptor Stimulation In Vivo and Ex Vivo in Newborn Rats

The study examined the effects of α_1A-adrenoceptor stimulation on chronotropic function of Langendorff-perfused isolated heart ex vivo and on cardiac chronotropy in vivo in 7-day-old rats. α_1A-Adrenergic receptor agonist A-61603 reduced heart chronotropy only in the whole organism. No chronotropic effects of selective stimulation of α_1A-adrenergic receptors on isolated hearts were observed in ex vivo experiments. These findings suggest that α_1A-adrenergic receptors are not implicated in HR regulation in newborn rats. Bradycardia induced by activation of these receptors in vivo is most likely associated with reflex influences on the heart and changes in the vascular tone in the whole organism.

Relation between spontaneous contraction and sarcoplasmic reticulum function in cultured neonatal rat cardiac myocytes

The relation between spontaneous contraction, Ca2+ oscillations, and sarcoplasmic reticulum (SR) function was studied in cultured neonatal rat cardiac myocytes. Spontaneous contraction and Ca2+ oscillations were irregular at day 2 of culture but became regular at day 6 of culture in neonatal rat cardiac myocytes. The rate of spontaneous contraction and the frequency of Ca2+ oscillations were decreased by verapamil and were abolished in the absence of extracellular Ca2+ at both day 2 and day 6 of culture. Ryanodine and thapsigargin increased the rate of contraction and the frequency of Ca2+ oscillations at day 2 of culture but did not affect contractions and Ca2+ oscillations at day 6 of culture. Ultrastructural observation showed that the structure of SR developed less at day 6 of culture. The present results suggest that spontaneous contraction and Ca2+ oscillations are due mainly to extracellular Ca2+ influx but not to Ca2+ release from SR in neonatal rat cardiac myocytes.

Negative chronotropic effect of botulinum toxin on neonatal rat cardiac myocytes

We demonstrated that botulinum neurotoxin attenuated the spontaneous beating rate of cultured cardiac myocytes. Primary cultured cardiac myocytes were prepared from the ventricles of neonatal Wistar rats (1-3 days old). On 7 days after cell seeding, botulinum toxin type A incorporated into liposomes was added to the culture medium. At a final concentration of 5.0 micrograms/ml, botulinum toxin markedly attenuated the beating rate of cardiac myocytes within 2-4 hours. These results demonstrated the effect of SNARE-complex proteins on the spontaneous beating of cardiac myocytes.