A. The relative effects of a PKA inhibitor PKI, carbachol (CCh), PDE inhibition (IBMX, milrinone) and isoproterenol (ISO) to alter the LCR period are linked to their effects to alter phospholamban (PLB) phosphorylation. The dashed line is the best fit least squares logarithmic function through the points: Y= −52.01 ln(X) + 107.11. B. The relative effects of PKI, CCh, PDE inhibition and ISO to alter the spontaneous cycle length over a wide range are linked to their effects on the LCR period. The best fit least squares linear functions (dashed line) through the points is: cycle length = 0.89 LCR period + 11.43 msec.