effective Ca2+ use during agonist-induced contraction

Agonist-induced Ca2+ influx may be more effective than depolarization-induced Ca2+ entry at raising cytoplasmic Ca2+ because of inhibition of a putative sarcoplasmic reticulum buffer barrier.

During the sustained phase of agonist-induced contraction, intracellular Ca2+ concentration is reported to be lower than during high K+ stimulation or in chemically skinned smooth muscle preparations, implying that a second regulatory system may exist.

Elevation of intracellular Ca2+ concentration is generally accepted as the main trigger for vascular smooth muscle contraction.