Reduced effects of BAY K 8644 on L-type Ca2+ current in failing human cardiac myocytes are related to abnormal adrenergic regulation

Abnormal L-type Ca2+ channel (LTCC, also named Cav1.2) density and regulation are important contributors to depressed contractility in failing hearts. The LTCC agonist BAY K 8644 (BAY K) has reduced inotropic effects on failing myocardium. We hypothesized that BAY K effects on the LTCC current ( ICaL) in failing myocytes would be reduced because of increased basal activity. Since support of the failing heart with a left ventricular assist device (LVAD) improves contractility and adrenergic responses, we further hypothesized that BAY K effects on ICaL would be restored in LVAD-supported failing hearts. We tested our hypotheses in human ventricular myocytes (HVMs) isolated from nonfailing (NF), failing (F), and LVAD-supported failing hearts. We found that 1) BAY K had smaller effects on ICaL in F HVMs compared with NF HVMs; 2) BAY K had diminished effects on ICaL in NF HVM pretreated with isoproterenol (Iso) or dibutyryl cyclic AMP (DBcAMP); 3) BAY K effects on ICaL in F HVMs pretreated with acetylcholine (ACh) were normalized; 4) Iso had no effect on NF HVMs pretreated with BAY K; 5) BAY K effects on ICaL in LVAD HVMs were similar to those in NF HVMs; 6) BAY K effects were reduced in LVAD HVMs pretreated with Iso or DBcAMP; 7) Iso had no effect on ICaL in LVAD HVMs pretreated with BAY K. Collectively, these results suggest that the decreased BAY K effects on LTCC in F HVMs are caused by increased basal channel activity, which should contribute to abnormal contractility reserve.

Intracellular sodium determines frequency-dependent alterations in contractility in hypertrophied feline ventricular myocytes

Hypertrophy and failure (H/F) in humans and large mammals are characterized by a change from a positive developed force-frequency relationship (+FFR) in normal myocardium to a flattened or negative developed force-frequency relationship (−FFR) in disease. Altered Ca2+ homeostasis underlies this process, but the role of intracellular Na+ concentration ([Na+]i) in H/F and frequency-dependent contractility reserve is unclear. We hypothesized that altered [Na+]i is central to the −FFR response in H/F feline myocytes. Aortic constriction caused left ventricular hypertrophy (LVH). We found that as pacing rate was increased, contraction magnitude was maintained in isolated control myocytes (CM) but decreased in LVH myocytes (LVH-M). Quiescent LVH-M had higher [Na+]i than CM (LVH-M 13.3 ± 0.3 vs. CM 8.9 ± 0.2 mmol/l; P < 0.001) with 0.5-Hz pacing (LVH-M 14.9 ± 0.5 vs. CM 10.8 ± 0.4 mmol/l; P < 0.001) but were not different at 2.5 Hz (17.0 ± 0.7 vs. control 16.0 ± 0.7 mmol/l; not significant). [Na+]i was altered by patch pipette dialysis to define the effect of [Na+]i on contraction magnitude and action potential (AP) wave shape at slow and fast pacing rates. Using AP clamp, we showed that LVH-M require increased [Na+]i and long diastolic intervals to maintain normal shortening. Finally, we determined the voltage dependence of contraction for Ca2+ current ( ICa)-triggered and Na+/Ca2+ exchanger-mediated contractions and showed that there is a greater [Na+]i dependence of contractility in LVH-M. These data show that increased [Na+]i is essential for maintaining contractility at slow heart rates but contributes to small contractions at fast rates unless rate-dependent AP shortening is prevented, suggesting that altered [Na+]i regulation is a critical contributor to abnormal contractility in disease.