Sarcoplasmic reticulum sarcolemma interactions and vascular smooth muscle tone

A characteristic of vascular smooth muscle cell morphology is a close apposition of its peripheral sarcoplasmic reticulum (SR) with the sarcolomma; this arrangement gives rise to important functional interactions whereby the peripheral SR regulates Ca2+ influx and vascular tone. We review here the key evidence supporting the following aspects of SR-sarcolemma interactions while establishing a conceptual framework encompassing (i) the SR ultrastructure and functions, (ii) the integration of the sarcolemmal Na+-Ca2+ exchanger and the peripheral SR in the mediation of a bidirectional Ca2+ exchange between the peripheral SR and the extracellular space, (iii) the existence of a higher myoplasmic free Ca2+ concentration [Ca2+](myo) in the subsarcolemmal space formed between the sarcolemma and the peripheral SR relative to the [Ca2+](myo) of the inner myoplasm in the resting smooth muscle cell, (iv) the division of the subsarcolemmal space into functional microdomains, (v) the existence of spontaneous localized bursts of Ca2+ release from the peripheral SR (Ca2+ sparks) towards the sarcolemma, (vi) the physiologicaI triggering of nonlocalized Ca2+ release from the peripheral SR by Ca2+ influx (Ca2+-induced Ca2+ release), and (vii) capacitative Ca2+ entry in vascular smooth muscle, We present an overview of the physiological and pathological implications of these interactions.