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 sarc olomma; this arrangement gives rise to important functional interactio ns 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 enc ompassing (i) the SR ultrastructure and functions, (ii) the integratio n of the sarcolemmal Na+-Ca2+ exchanger and the peripheral SR in the m ediation of a bidirectional Ca2+ exchange between the peripheral SR an d 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+](m yo) 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 th e peripheral SR (Ca2+ sparks) towards the sarcolemma, (vi) the physiol ogicaI 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 physi ological and pathological implications of these interactions.