Characterisation of inositol 1,4,5-trisphosphate binding sites in rabbit aortic smooth muscle

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Sarcoplasmic reticulum function in smooth muscle

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.

Pharmacological Characterization of Inositol 1,4,5-tris Phosphate Receptors in Human Platelet Membranes

The phosphatidylinositol (PI) hydrolysis signaling system has been shown to be altered in platelets of depressed and schizophrenic subjects. Inositol (1,4,5) trisphosphate (Ins(1,4,5)P₃), an integral component of the PI signaling system, mobilizes Ca²⁺ by activating Ins(1,4,5)P₃ receptors. To eventually investigate the role of Ins(1,4,5)P₃ receptors in depression and other mental disorders, we characterized [³H]Ins(1,4,5)P₃ binding sites in crude platelet membranes prepared from small amounts of blood obtained from healthy human control subjects. We found a single, saturable binding site for [³H]Ins(1,4,5)P₃ to crude platelet membranes, which is time dependent and modulated by pH, inositol phosphates, and heparin. Since cyclic adenosine monophosphate (cAMP) and Ca²⁺ have been shown to be important modulators in Ins(1,4,5)P₃ receptors, in the present study we also determined the effects of various concentrations of CaCI₂ and forskolin on Ins(1,4,5)P₃ binding to platelet membranes. CaCI₂ modulated [³H]Ins(1,4,5)P₃ binding sites in a biphasic manner: at lower concentrations it inhibited [³H]Ins(1,4,5)P₃ binding, whereas at higher concentrations, it stimulated [³H]Ins(1,4,5)P₃ binding. On the other hand, forskolin inhibited [³H]Ins(1,4,5)P₃ binding. Our results thus suggest that the pharmacological characteristics of [³H]Ins(1,4,5)P₃ binding to crude platelet membranes are similar to that of Ins(1,4,5)P₃ receptors; and that both Ca²⁺ and cAMP modulate [³H]Ins(1,4,5)P₃ binding in crude platelet membranes.

Effects of maturation and acute hypoxia on receptor-IP(3) coupling in ovine common carotid arteries

Whereas previous studies have established that many mechanisms mediating pharmacomechanical coupling are subject to regulation, evidence of physiological regulation of the coupling efficiency between receptor activation and second-messenger production is scarce. The present studies address the hypothesis that acute hypoxia and maturation can influence the mass of second-messenger production for each activated agonist-bound receptor ("receptor gain"). For this assessment, receptor density and agonist affinity values were used to calculate 5-hydroxytryptamine (5-HT) concentrations that would produce standardized numbers of bound receptors (8.5 fmol/mg protein) in each experimental group and thus minimize effects of age or hypoxia on receptor density or agonist affinity. After 3 min of exposure to these 5-HT concentrations, normoxic magnitudes of contraction were similar (as %potassium maxima) in fetal (50 +/- 14%) and adult (40 +/- 9%) arteries, but hypoxia (PO(2) approximately 9--12 Torr for 30 min) depressed contractile tensions with a significantly different time course and magnitude in fetal (30 +/- 10%) and adult (17 +/- 11%) arteries (P < 0.05). Basal inositol 1,4,5-trisphosphate (IP(3)) values (in pmol/mg protein) were significantly greater in fetal (94 +/- 16) than in adult (44 +/- 6) arteries, and integrated areas above baseline for the IP(3) time courses (in nmol-s/mg protein) were significantly greater in fetal than in adult arteries both in normoxic (14.3 +/- 1.8 vs. 9.1 +/- 1.6) and hypoxic (15.0 +/- 2.1 vs. 8.6 +/- 1.2) conditions (P < 0.05). Hypoxia altered the IP(3) time courses both in the fetus and the adult but had no significant effect on IP(3 )mobilization or receptor gain. These data demonstrate that for the 5-HT(2a) receptor predominant in this preparation, receptor gain can be experimentally determined, is not influenced by acute hypoxia, but is greater in fetal than in adult ovine carotid arteries.

Interactions between Ca(2+) and H(+) and functional consequences in vascular smooth muscle

Ca(2+) and H(+) ions can profoundly alter vascular tone. In many physiological and pathological processes, changes in the concentration of both ions occur. Thus, to understand the processes and mechanisms that modify force, it is necessary to understand what changes occur in these ions and, importantly, how they interact with each other. In this minireview, we highlight the quantitatively important mechanisms involved in the contractile responses of vascular tissues to pH change and discuss the cellular and molecular reasons underlying these responses.