Excitation-contraction coupling in vascular smooth muscle cells and perivascular nerve stimulation

Smooth muscle contraction depends largely on the increase of the cytoplasmic Ca2+ concentration. This change can be brought about by the opening of voltage-dependent Ca channels or receptor operated Ca channels. Although in some vascular smooth muscle cells the addition of noradrenaline does not appreciably change the membrane potential, it is observed that stimulation of the perivascular nerve fibres elicits excitatory junction potentials and action potentials. This difference between the action of exogenous and endogenous noradrenaline could be due to either the existence of intrajunctional gamma-receptors, which differ from the extrajunctional alpha-receptors or to the release from the nerve fibres of cotransmitters together with noradrenaline. It can be concluded that the clear distinction between electromechanical coupling and pharmacomechanical coupling only applies to some experimental conditions in vitro, but that the distinction cannot be used for conditions in vivo.