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Ion channels in smooth muscle

"Excitation-contraction coupling in smooth muscle..."

	Excitation-contraction coupling in smooth muscle is believed to occur by two mechanisms-electromechanical and pharmacomechanical coupling.
	Electromechanical coupling operates through changes in surface membrane potential; typically resting membrane potential= -40 to -70 mV.
	Primary drive for the rise in intracellular calcium is membrane depolarization, with the consequential opening of voltage operated calcium channels; neurotransmitters or hormones acting to depolarize the membrane will cause contraction while those producing membrane hyperpolarization will cause relaxation.
	Like cardiac muscle, the influx of Ca2+ likely causes release of Ca2+ from sarcoplasmic reticulum.

"Drugs that block calcium entry..."

	Drugs that block calcium entry through VOCC will inhibit electromechanical coupling-thus the use of calcium channel blocking agents to relax vascular smooth muscle, thus producing vasodilatation and a decrease in blood pressure.
	Cell-type dependent; for instance, in asthma, Ca2+ blocking drugs are not effective in promoting relaxation of muscle.
	Electromechanical coupling appears to play a predominant role in phasic smooth muscle in which the membrane potential often displays marked oscillations upon which are superimposed calcium spikes
	The plasma membranes contain numerous ion channels and the distribution and properties vary among different tissues, contributing to the diversity of smooth muscle.

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