title: Sarcoplasmic reticulum function in determining atrioventricular contractile differences in rat heart.
abstract: The relationships between the contractile characteristics and the sarcoplasmic reticulum (SR) function of rat atrial and ventricular trabeculae were compared. The isometric developed tension (DT) and the rates of contraction (+ dT/dt) and relaxation (-dT/dt) normalized to cross-sectional area were 3.7, 2.2, and 1.8 times lower, respectively, in intact atrial strips compared with ventricular strips, whereas + dT/dt and -dT/dt (normalized to DT) were 2.3 and 2.8 times higher, respectively, in atria. Atria exhibited a maximal potentiation of DT after shorter rest periods than ventricles and a lower reversal for prolonged rest periods. Caffeine-induced tension transients in saponin-permeabilized fibers suggested that the Ca2+ concentration released in atrial myofibrils reached a lower maximum and decayed more slowly than in ventricular preparations. However, the tension-time integrals indicated an equivalent capacity of sequestrable Ca2+ in SR from both tissues. In atrial, as in ventricular myocardium, the SR Ca2+ uptake was more efficiently supported by ATP produced by the SR-bound MM form of creatine kinase (CK; MM-CK) than by externally added ATP, suggesting a tight functional coupling between the SR Ca2+ adenosinetriphosphatase (ATPase) and MM-CK. The maximal rate of oxalate-supported Ca2+ uptake was two times higher in atrial than in ventricular tissue homogenates. The SR Ca(2+)-ATPase 2a mRNA content normalized to 18S RNA was 38% higher in atria than in ventricles, whereas the amount of mRNA encoding the alpha-myosin heavy chain, calsequestrin, and the ryanodine receptor was similar in both tissues. Thus a lower amount of readily releasable Ca2+ together with a faster uptake rate may partly account for the shorter time course and lower tension development in intact atrial myocardium compared with ventricular myocardium.
authors.affiliation: Department of Pathological Physiology, Medical Faculty, University of Tartu, Estonia.
authors.forename: A
authors.initials: A
authors.lastname: Minajeva
authors.forename: A
authors.initials: A
authors.lastname: Kaasik
authors.forename: K
authors.initials: K
authors.lastname: Paju
authors.forename: E
authors.initials: E
authors.lastname: Seppet
authors.forename: A M
authors.initials: AM
authors.lastname: Lompré
authors.forename: V
authors.initials: V
authors.lastname: Veksler
authors.forename: R
authors.initials: R
authors.lastname: Ventura-Clapier
mesh_terms: D000818:Animals; D016275:Atrial Function; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000096:biosynthesis; D002155:Calsequestrin / Q000096:biosynthesis; D003238:Connective Tissue / Q000502:physiology; D004558:Electric Stimulation; D005260:Female; D006321:Heart / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000502:physiology; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism; D018995:Myosin Heavy Chains / Q000096:biosynthesis; D012333:RNA, Messenger / Q000096:biosynthesis; D012337:RNA, Ribosomal, 18S / Q000096:biosynthesis; D051381:Rats; D017208:Rats, Wistar; D019837:Ryanodine Receptor Calcium Release Channel / Q000096:biosynthesis; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000502:physiology*; D014158:Transcription, Genetic; D016276:Ventricular Function
chemical_list: D002155:Calsequestrin; D012333:RNA, Messenger; D012337:RNA, Ribosomal, 18S; D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D018995:Myosin Heavy Chains; D000252:Calcium-Transporting ATPases; D002118:Calcium