Inotropic response of cardiac ventricular myocytes to beta-adrenergic stimulation with isoproterenol exhibits diurnal variation: involvement of nitric oxide.
BACKGROUND
Although >10% of cardiac gene expression displays diurnal variations, little is known of their impact on excitation-contraction coupling.


OBJECTIVE
To determine whether the time of day affects excitation-contraction coupling in rat ventricles.


RESULTS
Left ventricular myocytes were isolated from rat hearts at 2 opposing time points, corresponding to the animals resting or active periods. Basal contraction and [Ca(2+)](i) was significantly greater in myocytes isolated during the resting versus active periods (cell shortening 12.4+/-0.3 versus 11.0+/-0.2%; P<0.05 and systolic [Ca(2+)](i) 422+/-12 versus 341+/-9 nmol/L; P<0.01. This corresponded to a greater sarcoplasmic reticulum (SR) Ca(2+) load (672+/-20 versus 551+/-13 nmol/L P<0.001). The increase in systolic [Ca(2+)](i) in response to isoproterenol (>3 nmol/L) was also significantly greater in resting versus active period myocytes, reflecting a greater SR Ca(2+) load at this time. This diurnal variation in response of Ca(2+)-homeostasis to isoproterenol translated to a greater incidence of arrhythmic activity in resting period myocytes. Inhibition of neuronal NO synthase during stimulation with isoproterenol, further increased systolic [Ca(2+)](i) and the percentage of arrhythmic myocytes, but this effect was significantly greater in active period versus resting period myocytes. Quantitative RT-PCR analysis revealed a 2.65-fold increase in neuronal NO synthase mRNA levels in active over resting period myocytes (P<0.05).


CONCLUSIONS
The threshold for the development of arrhythmic activity in response to isoproterenol is higher during the active period of the rat. We suggest this reflects a reduction in SR Ca(2+) loading and a diurnal variation in neuronal NO synthase signaling.
Collins
Helen E
HE
Department of Cardiovascular Sciences, University of Leicester, Glenfield General Hospital, Leicester LE3 9QP, United Kingdom.
Rodrigo
Glenn C
GC
D000318:Adrenergic beta-Agonists / Q000009:adverse effects / Q000494:pharmacology*; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000139:chemically induced / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects; D002304:Cardiac Pacing, Artificial; D002940:Circadian Rhythm; D004305:Dose-Response Relationship, Drug; D056966:Excitation Contraction Coupling / Q000187:drug effects*; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D006352:Heart Ventricles / Q000187:drug effects / Q000378:metabolism; D006706:Homeostasis; D007545:Isoproterenol / Q000009:adverse effects / Q000494:pharmacology*; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects*; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D009569:Nitric Oxide / Q000378:metabolism*; D019001:Nitric Oxide Synthase / Q000235:genetics / Q000378:metabolism; D052248:Nitric Oxide Synthase Type I; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D015854:Up-Regulation; D016277:Ventricular Function, Left / Q000187:drug effects; D017725:Ventricular Pressure / Q000187:drug effects
D000318:Adrenergic beta-Agonists; D020746:Calcium Channels, L-Type; D012333:RNA, Messenger; D009569:Nitric Oxide; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; C496322:Nos1 protein, rat; D007545:Isoproterenol; D002118:Calcium