Biphasic effects of hyposmotic challenge on excitation-contraction coupling in rat ventricular myocytes

Abstract

The effects of short (1 min) and long (7-10 min) exposure to hyposmotic solution on excitation-contraction coupling in rat ventricular myocytes were studied. After short exposure, the action potential duration at 90% repolarization (APD(90)), the intracellular Ca(2+) concentration ([Ca(2+)](i)) transient amplitude, and contraction increased, whereas the L-type Ca(2+) current (I(Ca, L)) amplitude decreased. Fractional sarcoplasmic reticulum (SR) Ca(2+) release increased but SR Ca(2+) load did not. After a long exposure, I(Ca,L), APD(90), [Ca(2+)](i) transient amplitude, and contraction decreased. The abbreviation of APD(90) was partially reversed by 50 microM DIDS, which is consistent with the participation of Cl(-) current activated by swelling. After 10-min exposure to hyposmotic solution in cells labeled with di-8-aminonaphthylethenylpyridinium, t-tubule patterning remained intact, suggesting the loss of de-t-tubulation was not responsible for the fall in I(Ca,L). After long exposure, Ca(2+) load of the SR was not increased, and swelling had no effect on the site-specific phosphorylation of phospholamban, but fractional SR Ca(2+) release was depressed. The initial positive inotropic response to hyposmotic challenge may be accounted for by enhanced coupling between Ca(2+) entry and release. The negative inotropic effect of prolonged exposure can be accounted for by shortening of the action potential duration and a fall in the I(Ca,L) amplitude.

Critical Evaluation of Cardiac Ca2+-ATPase Phosphorylation on Serine 38 Using a Phosphorylation Site-specific Antibody

“…In previous studies, Ser-16 phosphorylation of phospholamban in myocytes following similar interventions was quantified at 8.5 pmol/1,000 cells (46), indicating the presence of at least 85 pmol of phospholamban in the 10,000 cells of the present study. As phospholamban and SERCA are expressed in similar amounts in cardiac muscle (2 phospholamban per SERCA, Ref.…”

“…In each case interventions that activate CaMKII have been deployed, using the optimal conditions described in the literature (26,(45)(46)(47). In experiments using cardiac SR vesicles and those using isolated rat myocytes, Thr-17 phosphorylation of phospholamban has demonstrated high levels of CaMKII activity focused on SR targets.…”

Hyposmotic challenge modulates function of L-type calcium channel in rat ventricular myocytes through protein kinase C

“…For example, I Ca,L increases in neonatal rat ventricular myocytes [14] and rabbit sinoatrial node and atrial cells [12,13] , remains unaltered in canine ventricular myocytes [17] , decreases or remains unaltered in guinea-pig ventricular myocytes [10,15] . Brette et al had reported that in rat ventricular myocytes osmotic swelling decreases I Ca,L [16] . On the other hand, Li et al had found that in rabbit ventricular myocytes hyposmotic swelling induces biphasic changes, an increase followed by decrease, of I Ca,L under perforated whole-cell patch-clamp conditions [18] .…”

“…Some authors have even demonstrated biphasic effects on I Ca,L , an increase followed by a more sustained decrease [18] . Previous reports indicated that osmotic swelling increases I Ca,L in neonatal rat ventricular myocytes [14] , but decreases I Ca,L in adult rat ventricular myocytes [16] . But these reports did not explore the mechanism underlying the changes of I Ca,L .…”

“…However, the effect of cell swelling on cardiac I Ca,L remains controversial. For example, some studies have shown clear increases in I Ca,L [12][13][14] , while others have shown a decrease in I Ca,L [15,16] or no effect [10,17] . Some authors have even demonstrated biphasic effects on I Ca,L , an increase followed by a more sustained decrease [18] .…”