dude:VCU07 said:The release of calcium from the SR should depolarize the cells and thus make them more negative eventually leading to an action potential. This should increase the firing rate and cause muscle contraction. You must remember that the heart is myogenic (although it can still be under sympathic and parasympathetic control). There are two mechanisms by which the APs are generated in the SA node and then another, different mechanism by which the APs are generated in the AV Node, Purkinje Fibers, and Ventricular Fibers. The only way I can think of to decrease the Heart contraction if by activating the parasyms.
oh yeah, so Ca increase does not reduce firing of a heart muscle cell... could you please describe where you may have read this?whatzcool said:How does calcium increase the threshold potential (ie, make it less negative) and thus reduce firing of a heart muscle cell?
So, people use Ca to treat hyperkalemia, but what makes Ca to raise threshold instead of resting membrane potential?Intravenous calcium is the first-line drug in the emergency management of hyperkalemia. The mutually antagonistic effects of Ca2+ and K+ on the myocardium and the protective role of Ca2+ in hyperkalemia have long been known. Calcium raises the action potential threshold and reduces excitability without changing the resting membrane potential. By restoring the difference between resting and threshold potentials, Ca2+ reverses the depolarization blockade due to hyperkalemia.