The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a “one model fits all” approach to this subject, we have tried to synthesize conclusions wherever possible.

The mechanism underlying spontaneous pacemaker potential in the uterus is not clearly understood. Several spontaneously active smooth muscles have interstitial cells of Cajal (ICCs) or ICC-like cells. We therefore examined cells from freshly dispersed uterine muscle strips (from pregnant human and rat myometrium) and in situ uterine preparations to determine the cell types present. Both preparations revealed numerous ICC-like cells; they were multipolar, with spider-like projections and enlarged central regions. These cells were readily distinguished from uterine myocytes by their morphology and ultrastructure, i.e., no myofilaments, numerous mitochondria, caveolae, and filaments. In addition, the ICC-like cells were noncontractile. These cells were negative to c-kit, a classic marker for ICCs. They stained positive for the intermediate filament, vimentin, a marker for cells of mesenchymal origin but not differentiated myocytes. The ICC-like cells had a more or less stable resting membrane potential of -58+/-7 mV compared with smooth-muscle cells, -65+/-13 mV, and produced outward current in response to voltage clamp pulses. However, in contrast with uterine myocytes, inward currents were not observed. This is the first description of ICC-like cells in myometrium and their role in the uterus is discussed, as possible inhibitors of intrinsic smooth-muscle activity.

In this review we discuss our current understanding of the cellular basis of uterine contractility, highlighting those areas requiring further study. It is clear that the basic processes of excitation‐contraction coupling lie within the myometrial cell, and that these may be modified by agonists. Pacemaker acitivity, however, remains a mystery. The contribution of extracellular calcium entry to contraction is shown to be vital, whilst the role of the sarcoplasmic reticulum remains controversial. Much current experimental focus is on pathways controlling and regulating contraction, and we discuss sensitisation mechanisms and question their role in intact uterine preparations.

The microcirculation is the site of gas and nutrient exchange. Control of central or local signals acting on the myocytes, pericytes and endothelial cells within it, is essential for health. Due to technical problems of accessibility, the mechanisms controlling Ca2+ signalling and contractility of myocytes and pericytes in different sections of microvascular networks in situ have not been investigated. We aimed to investigate Ca2+ signalling and functional responses, in a microcirculatory network in situ. Using live confocal imaging of ureteric microvascular networks, we have studied the architecture, morphology, Ca2+ signalling and contractility of myocytes and pericytes. Ca2+ signals vary between distributing arcade and downstream transverse and precapillary arterioles, are modified by agonists, with sympathetic agonists being ineffective beyond transverse arterioles. In myocytes and pericytes, Ca2+ signals arise from Ca2+ release from the sarcoplasmic reticulum through inositol 1,4,5-trisphosphate-induced Ca2+ release and not via ryanodine receptors or Ca2+ entry into the cell. The responses in pericytes are less oscillatory, slower and longer-lasting than those in myocytes. Myocytes and pericytes are electrically coupled, transmitting Ca2+ signals between arteriolar and venular networks dependent on gap junctions and Ca2+ entry via L-type Ca2+ channels. Endothelial Ca2+ signalling inhibits intracellular Ca2+ oscillations in myocytes and pericytes via L-arginine/nitric oxide pathway and intercellular propagating Ca2+ signals via EDHF. Increases of Ca2+ in pericytes and myocytes constrict all vessels except capillaries. These data reveal the structural and signalling specializations allowing blood flow to be regulated by myocytes and pericytes.

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In excitable tissues the refractory period is a critical control mechanism preventing hyperactivity and undesirable tetani, by preventing subsequent stimuli eliciting action potentials and Ca2+ entry. In ureteric smooth muscle, peristaltic waves that occur as invading pacemaker potentials produce long-lasting action potentials (300-800 ms) and extraordinarily long (more than 10 s) refractory periods, which prevent urine reflux and kidney damage. For smooth muscles neither the mechanisms underlying the refractory period nor the link between excitability and refractoriness are properly understood. Here we show that a negative feedback process, which depends on Ca2+ loading the sarcoplasmic reticulum (SR) during the action potential and on the subsequent activation of local releases of Ca2+ from the SR (sparks), stimulating plasmalemmal Ca2+-sensitive K+ (BK) channels, determines the refractory period of the action potential. As sparks gradually reduce the Ca2+ load in the SR, electrical inhibition is released, the refractory period is terminated and peristaltic contractions occur again. The refractory period can be manipulated, for example from 10 s to 100 s, by altering the Ca2+ content of the SR or release mechanism or by inhibiting BK channels. This insight into the control of excitability and hence function provides a focus for therapies directed at pathologies of smooth muscle.

2+ -independent increase in the relaxation rate of contraction, associated with acceleration of MLC dephosphorylation, which was sensitive to calyculin A. These data show for the first time that: (1) Rho-kinase has major effects on Ca 2+ signalling associated with the action potential, (2) this effect is species dependent and (3) Rho-kinase controls relaxation of phasic contraction of myogenic origin. Thus Rho-kinase can modulate phasic smooth muscle in the absence of agonist, and the mechanisms are both Ca 2+ -dependent, involving ion channels, and Ca 2+ -independent, involving MLC phosphorylation activity.

Recent work has indicated that smooth muscle force production may be influenced by pathways not dependent upon the Ca2+-calmodulin phosphorylation of light chains. Few studies, however, have examined the importance of these pathways in intact muscles that contract phasically rather than tonically. Therefore, to determine whether the Ca2+-independent Rho-A and associated kinase (ROK) pathway can affect contractions of the intact human myometrium, we used Y-27632 to inhibit ROK. Three types of contractile activity were examined: spontaneous and those elicited by oxytocin and by depolarisation by high K+. Y-27632 decreased force significantly under all three conditions, without changing intracellular [Ca2+]. However, the effects on force were only large when the uterus was producing force tonically rather than phasically. This suggests that the Rho-A-ROK pathway may not be a potent modulator of force in the human myometrium under physiological conditions.