sympathetic stimulation results in both positive inotropic and chronotropic effect. Here is more detail for those who would like to know.
Explanation of the positive inotropic effect of symp. stimulation:
Norepinephrine released by axon terminals of sympathetic neurons stimulates B1 adrenergic receptros. The G-pro increases cAMP levels in the cell, whirch causes a positive inotropic effects by two pathways. First, The opening time of voltage gated calcium channels increases, you get more calcium in the cell, and a stronger contraction (positive inotropic effect). The second pathway is inhibition of phospholamban by phosphorylation. The inhibition of phospholamban results in a higher Ca2+ ATPase activity, which restores the calcium into the Sarcoplasmic reticulum faster. A faster restoration of calcium means a shorter Ca-troponin binding, which will result in a longer relaxation phase. Longer relaxation leads to a larger EDV, and you get a stronger contraction (positive inotropic effect) based on the Starling curve.
Explanation of the positive chronotropic effect of symp. stimulation:
As mentioned above, you get higher opening time of Ca2+ channels by cAMP activity. cAMP also increases the inward funny current of pacemaker cells by phosphorylating HCN channels and increasing their opening time. Therefore, the threshold of Action potential in pacemaker cells is achieved faster--->faster heart rate --> positive chronotropic effect.
Edit: Jaskiran, you are a Hero (for pulling up an old resolved thread to post a wrong answer
)!
