Question

I need to provide proof that these statements are FALSE. But I don't always understand why they are false.

1.

"drugs that block L-type Ca channels, reduce the contraction-efficiency of cardiac muscle, but not of skeletal muscles."

(dont the skeletal muscles just need the tiniest bit of Ca to use the DHP channels?)

2.

"the length of a skeletal muscle twitch is about 10-100 ms"

(is this maybe for all muscle types? and not just for skeletal muscles?)

thx in advance!

Answer 1

L-type calcium channels are responsible for the excitation-contraction coupling of skeletal, smooth, cardiac muscle, and for aldosterone secretion in endocrine cells of the adrenal cortex.They are also found in neurons, and with the help of L-type calcium channels in endocrine cells, they regulate neurohormones and neurotransmitters. They have also been seen to play a role in gene expression, mRNA stability, neuronal survival, ischemic-induced axonal injury, synaptic efficacy, and both activation and deactivation of other ion channels.

In cardiac myocytes, the L-type calcium channel passes inward Ca2+ current and triggers calcium release from the sarcoplasmic reticulum by activating ryanodine receptor 2 (RyR2) (calcium-induced-calcium-release). Phosphorylation of these channels increases their permeability to calcium and increases the contractility of their respective cardiac myocytes.

L-type calcium channel blocker drugs are used as cardiac antiarrhythmics or antihypertensives, depending on whether the drugs have higher affinity for the heart (the phenylalkylamines, like verapamil), or for the blood vessels (the dihydropyridines, like nifedipine).

In skeletal muscle, there is a very high concentration of L-type calcium channels, situated in the T-tubules. Muscle depolarization results in large gating currents, but anomalously low calcium flux, which is now explained by the very slow activation of the ionic currents. For this reason, little or no Ca2+ passes across the T-tubule membrane during a single action potential.

So when blocked both contraction will be reduced