Sites of Control of Cardiac Force. 2 examples of how Contractility is changed by by phosphorylation events L-type Channels. (a)Action of norepinephrine...activation of B adrenergic receptor...increases cAMP...PKA produced...PKA phosphorylates L-Channels and increases their Calcium conductance...increasing Ca influx...increasing force (b)Action of acetylcholine...activation of muscarinic receptor...increases cGMP...results in phosphorylating a different site on L-Channel,which decreases Ca influx and thus decreases force. force is varied in cardiac muscle by altering the Calcium level in myocytes