UC Davis

β-adrenergic (β-AR) signaling is essential for the adaptation of the heart to exercise and stress. Chronic stress leads to the activation of Ca²⁺/calmodulin-dependent kinase II (CaMKII) and protein kinase D (PKD). Unlike CaMKII, the effects of PKD on excitation-contraction coupling (ECC) remain unclear. To elucidate the mechanisms of PKD-dependent ECC regulation, we used hearts from cardiac-specific PKD1 knockout (PKD1 cKO) mice and wild-type (WT) littermates. We measured calcium transients (CaT), Ca²⁺ sparks, contraction and L-type Ca²⁺ current in paced cardiomyocytes under acute β-AR stimulation with isoproterenol (ISO; 100 nM). Sarcoplasmic reticulum (SR) Ca²⁺ load was assessed by rapid caffeine (10 mM) induced Ca²⁺ release. Expression and phosphorylation of ECC proteins phospholambam (PLB), troponin I (TnI), ryanodine receptor (RyR), sarcoendoplasmic reticulum Ca²⁺ ATPase (SERCA) were evaluated by western blotting. At baseline, CaT amplitude and decay tau, Ca²⁺ spark frequency, SR Ca²⁺ load, L-type Ca²⁺ current, contractility, and expression and phosphorylation of ECC protein were all similar in PKD1 cKO vs. WT. However, PKD1 cKO cardiomyocytes presented a diminished ISO response vs. WT with less increase in CaT amplitude, slower [Ca²⁺]_i decline, lower Ca²⁺ spark rate and lower RyR phosphorylation, but with similar SR Ca²⁺ load, L-type Ca²⁺ current, contraction and phosphorylation of PLB and TnI. We infer that the presence of PKD1 allows full cardiomyocyte β-adrenergic responsiveness by allowing optimal enhancement in SR Ca²⁺ uptake and RyR sensitivity, but not altering L-type Ca²⁺ current, TnI phosphorylation or contractile response. Further studies are necessary to elucidate the specific mechanisms by which PKD1 is regulating RyR sensitivity. We conclude that the presence of basal PKD1 activity in cardiac ventricular myocytes contributes to normal β-adrenergic responses in Ca²⁺ handling.