The mechanism responsible for sustained vascular smooth muscle contraction is

Latch bridge mechanism: - Once smooth muscle has developed full contraction, the amount of continuing excitation can usually be reduced to far less than the initial level even though the muscle maintains its full force of contraction. Fuher the energy consumed to maintain contraction is often miniscule, sometimes as little as 1/ 300 the energy required for comparable sustained skeletal muscle contraction. This mechanism is k/a 'latch' mechanism. The impoance of latch mechanism is that it can maintain prolonged toxic contraction in smooth muscle for hours with little use of energy. Little continued excitatory signal is required from nerve fibres or hormonal source.