Histamine-induced calcium oscillations in human vascular smooth muscle: temporal sequence and spatial organization in single cells

Integrin mobilizes intracellular Ca(2+) in renal vascular smooth muscle cells

Peptides with the Arg-Gly-Asp (RGD) motif induce vasoconstriction in rat afferent arterioles by increasing the intracellular Ca(2+) concentration ([Ca(2+)](i)) in vascular smooth muscle cells (VSMC). This finding suggests that occupancy of integrins on the plasma membrane of VSMC might affect vascular tone. The purpose of this study was to determine whether occupancy of integrins by exogenous RGD peptides initiates intracellular Ca(2+) signaling in cultured renal VSMC. When smooth muscle cells were exposed to 0.1 mM hexapeptide GRGDSP, [Ca(2+)](i) rapidly increased from 91 +/- 4 to 287 +/- 37 nM and then returned to the baseline within 20 s (P < 0.05, 34 cells/5 coverslips). In controls, the hexapeptide GRGESP did not trigger Ca(2+) mobilization. Local application of the GRGDSP induced a regional increase of cytoplasmic [Ca(2+)](i), which propagated as Ca(2+) waves traveling across the cell and induced a rapid elevation of nuclear [Ca(2+)](i). Spontaneous recurrence of smaller-amplitude Ca(2+) waves were found in 20% of cells examined after the initial response to RGD-containing peptides. Blocking dihydropyridine-sensitive Ca(2+) channels with nifedipine or removal of extracellular Ca(2+) did not inhibit the RGD-induced Ca(2+) mobilization. However, pretreatment of 20 microM ryanodine completely eliminated the RGD-induced Ca(2+) mobilization. Anti-beta(1) and anti-beta(3)-integrin antibodies with functional blocking capability simulate the effects of GRGDSP in [Ca(2+)](i). Incubation with anti-beta(1)- or beta(3)-integrin antibodies inhibited the increase in [Ca(2+)](i) induced by GRGDSP. We conclude that exogenous RGD-containing peptides induce release of Ca(2+) from ryanodine-sensitive Ca(2+) stores in renal VSMC via integrins, which can trigger cytoplasmic Ca(2+) waves propagating throughout the cell.

Mechanisms underlying phasic contractions of pregnant rat myometrium stimulated with aluminum fluoride

OBJECTIVE

The mechanisms underlying phasic myometrial contractions are unknown at this time. Phasic contractions, however, are characterized by repetitive cycles of elevated intracellular calcium (i.e., calcium oscillations). These studies were performed to test the hypothesis that mechanisms underlying phasic myometrial contractions are similar to those producing classic cytosolic calcium oscillations.

STUDY DESIGN

Uterine tissue was obtained from pregnant Sprague-Dawley rats (i.e., day 18 to 22 of gestation). In vitro isometric contraction studies were performed with longitudinal strips of myometrial tissue; computer-digitized data were analyzed for contraction area and normalized for tissue cross-section area. Dose-response studies were performed with aluminum fluoride and various inhibitors of cytosolic calcium oscillations.

RESULTS

Aluminum fluoride stimulated a significant increase in phasic contractions. In contrast, the addition of 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (an inhibitor of phosphoinositide-specific phospholipase C), adenine (an inhibitor of calcium-induced calcium release), and phorbol 12,13-dibutyrate (an activator of protein kinase C) resulted in significant suppression of aluminum fluoride-stimulated contractions. Similarly, nifedipine (an L-type calcium channel blocker) and removal of extracellular calcium significantly inhibited phasic myometrial contractions.

CONCLUSIONS

These studies have confirmed that phosphoinositide-specific phospholipase C, calcium-induced calcium release, protein kinase C, and transmembrane calcium influx are important components of the intracellular calcium oscillator that generates agonist-stimulated phasic contractions of pregnant myometrial tissue.

The time course of intracellular calcium movements in single human umbilical vein smooth muscle cells

Intracellular Ca2+ ([Ca2+]i) was measured in single isolated human umbilical vein smooth muscle cells. Stimulation with histamine, in the absence of external Ca2+, mobilised Ca2+ from intracellular stores. When repeated brief applications of agonist were used, the time to onset, amplitude and rate of rise of the Ca2+ transients were found to change. Two components could often be discerned in the rising phase of the transients, an initial slow "pacemaker" and a second faster and larger component. Following the first histamine-activated transient the basal level of [Ca2+]i was invariably lower than that prior to stimulation. This lower value was maintained whilst the cell remained in Ca(2+)-free solution, but could be returned to a higher level if the cell was exposed to external Ca2+. When the mobilisation of the intracellular store was reduced to undetectable levels, re-exposure to Ca(2+)-containing medium reactivated responses. In the absence of external Ca2+, continuous application of histamine activated a series of transient increases in intracellular Ca2+, which decreased progressively in amplitude and rate of rise. The interval between transients also increased. These findings are discussed in terms of the activation of inositol trisphosphate-sensitive intracellular Ca2+ stores and their sensitivity to cytoplasmic Ca2+ and intrasarcoplasmic reticulum Ca2+.

Classes and mechanisms of calcium waves

The best known calcium waves move at about 5-30 microns/s (at 20 degrees C) and will be called fast waves to distinguish them from slow (contractile) ones which move at 0.1-1 microns/s as well as electrically propagated, ultrafast ones. Fast waves move deep within cells and seem to underlie most calcium signals. Their velocity and hence mechanism has been remarkably conserved among all or almost all eukaryotic cells. In fully active (but not overstimulated) cells of all sorts, their mean speeds lie between about 15-30 microns/s at 20 degrees C. Their amplitudes usually lie between 3-30 microM and their frequencies from one per 10-300 s. They are propagated by a reaction diffusion mechanism governed by the Luther equation in which Ca2+ ions are the only diffusing propagators, and calcium induced calcium release, or CICR, the only reaction; although this reaction traverses various channels which are generally modulated by IP3 or cADPR. However, they may be generally initiated by a second, lumenal mode of CICR which occurs within the ER. Moreover, they are propagated between cells by a variety of mechanisms. Slow intracellular waves, on the other hand, may be mechanically propagated via stretch sensitive calcium channels.