Properties of voltage‐activated [Ca²⁺]_i transients in single smooth muscle cells isolated from pregnant rat uterus

Abstract

The intracellular calcium concentration ([Ca2+]i) was measured at 35 °C using the fluorescent indicator indo‐1 in patch‐clamped, single uterine myocytes from pregnant rats to investigate the relationship between depolarization, Ca2+ current (ICa) and [Ca2+]i. Membrane depolarization activated ICa and produced a [Ca2+]i transient. The rapid increase in [Ca2+]i occurred at the same time as the inward ICa. Both ICa and the increase in [Ca2+]i were abolished by nifedipine (10 μm). When the membrane potential was held at ‐80 mV the threshold depolarization for an increase in [Ca2+]i was about ‐55 to ‐50 mV. As the magnitude of the depolarization was increased to about 0 mV there was an increase in the size of both ICa and the increase in [Ca2+]i. As the magnitude of the depolarization was further increased both ICa and the [Ca2+]i increase declined. When the depolarizing pulses were applied at 3 Hz to mimic normal action potentials then the individual [Ca2+]i transients did not fully relax and a tetanic rise of [Ca2+]i was observed. Under these conditions, there was not a simple relationship between the magnitude of the Ca2+ response and Ca2+ entry. When pairs of depolarizing pulses were applied, the increase in [Ca2+]i produced by the second pulse was larger (in relation to the magnitude of the L‐type Ca2+ current) than that produced by the first pulse. This facilitation was abolished by both ryanodine and cyclopiazonic acid suggesting a role for release from intracellular stores. We conclude that the L‐type Ca2+ current is the major source of Ca2+ ions entering the cell to produce the [Ca2+]i transient on depolarization. The magnitude of the increase in [Ca2+]i may, however, be amplified by Ca2+‐induced Ca2+ release.

Role of T-type Ca2+ Channels in the Spontaneous Phasic Contraction of Pregnant Rat Uterine Smooth Muscle

“…The results are consistent with a previous study that mibefradil inhibited the frequency as well as amplitude of uterine contractility (Asokan et al, 2002 ]i and force. According to the previous report, high K ＋ -induced contraction is due to Ca 2＋ influx through L-type Ca 2＋ channel by membrane depolarization (Shmigol et al, 1998;Coleman et al, 2000). In the present study, we also showed that 1 μM nifedipine, L-type Ca 2＋ channel blocker, completely inhibited high K ＋ -induced contraction.…”

Ca2+ entry following P2X receptor activation induces IP3 receptor‐mediated Ca2+ release in myocytes from small renal arteries

“…Electrical stimulation of these SMCs, in which close proximity of VGCCs and RyRs within the caveolar domains was confirmed with 3D-immunofluorescence and electron microscopy (Moore et al, 2004), triggers an abrupt RyR-mediated Ca 2+ release at multiple sub-PM regions ('hot spots ', Ohi et al, 2001;Morimura et al, 2006). The ability of Ca 2+ entering the SMC via VGCCs to induce RyR-mediated Ca 2+ release has also been demonstrated in voltage-clamp experiments performed on SMCs from pregnant myometrium (Shmigol et al, 1998), ileum (Kohda et al, 1997), mesenteric artery (Bolton and Gordienko, 1998), cerebral arteries (Kamishima and McCarron, 1997) and portal vein (Coussin et al, 2000). However, other research groups working on different or even the same SMC type failed to detect any evidence of CICR (Fleischmann et al, 1996;Kamishima and McCarron, 1996;Bradley et al, 2002).…”

“…Although activation of the SR Ca 2+ release by Ca 2+ entering the cell via VGCCs has been demonstrated in voltage-clamp experiments performed on different types of visceral and vascular SMCs (Kamishima and McCarron, 1997;Kohda et al, 1997;Bolton and Gordienko, 1998;Shmigol et al, 1998;Coussin et al, 2000), there is a number of studies (e.g. Bradley et al, 2002;2004) (Bradley et al, 2004).…”

“…In this study we have demonstrated that in RVSMCs 2-APB exerts no effect on the Ca 2+ entry mechanisms induced by P2X receptor activation and the SR calcium load ( Figure 5). Because the effect of 2-APB on ab-meATP-induced [Ca 2+ ]i transient was significantly attenuated in the presence of nicardipine, we concluded that in RVSMCs IP3R-mediated Ca 2+ release following P2X receptor stimulation is activated mainly by Ca 2+ entering the cell via VGCCs.Although activation of the SR Ca 2+ release by Ca 2+ entering the cell via VGCCs has been demonstrated in voltage-clamp experiments performed on different types of visceral and vascular SMCs (Kamishima and McCarron, 1997;Kohda et al, 1997;Bolton and Gordienko, 1998;Shmigol et al, 1998;Coussin et al, 2000), there is a number of studies (e.g. Bradley et al, 2002;2004) (Bradley et al, 2004).…”

Mathematical model of excitation-contraction in a uterine smooth muscle cell

“…Stimulation of isolated myocytes using voltage pulses revealed the current-voltage relationships of the pregnant myometrial SMCs in rats (18,34) and in humans (7,37). Application of single voltage pulses demonstrated that the Ca 2ϩ current (I Ca ) through L-type VOCCs significantly increased C Ca,i , whereas repetitive stimulation with pulse trains revealed that both VOCC opening and Ca 2ϩ -induced Ca 2ϩ release (CICR) from the sarcoplasmic reticulum (SR) are responsible for the increased C Ca,i (18). The entry current following depolarization of rat myometrial cells comprises two components, which were identified based on differences in their activation and inactivation properties as well as in their kinetics.…”