Capacitative Ca2+ entry associated with α1-adrenoceptors in rat aorta
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In rat aorta, depletion of internal Ca2+ stores by addition of noradrenaline (1μM) induces a biphasic response (an initial phasic response and a tonic one) mediated by two different intracellular Ca2+ pools. This response cannot be repeated, suggesting a depletion of internal Ca2+ stores sensitive to noradrenaline. In absence of the agonist, this depletion is the signal for the entry of extracellular Ca2+, not only to refill the stores but also, under our experimental conditions, to activate the contractile proteins thus inducing an increase in the resting tone (IRT) that constitutes functional evidence of this Ca2+ entry. The ionic channels involved in the mechanism of the IRT have been studied in the present work.
The fact that the addition of nimodipine (10–15–
10–11M) selectively inhibits the IRT suggests that this mechanical response is mediated by Ca2+ influx through dihydropyridine-sensitive Ca2+ channels. Moreover, the inhibitory action of nimodipine is attenuated by glibenclamide (10μM). Cromakalim (10–10–10–6M) also inhibits the IRT concentration dependently, and this inhibition is antagonized by glibenclamide (10μM). These results relate the ATP-dependent K+ channels to the mechanism of the IRT.
The refilling of the two internal Ca2+ compartments sensitive to noradrenaline was, like the IRT, altered in presence of the compounds tested, since the subsequent contractile response to noradrenaline was decreased. The present results suggest that nimodipine treatment inhibits the refilling of the Ca2+ compartment responsible for the tonic contraction induced by noradrenaline in Ca2+-free medium, whereas the refilling of the Ca2+ pool responsible for the phasic response to noradrenaline remained unaltered. Both the phasic and tonic responses to noradrenaline in Ca2+-free medium decreased after treatment with cromakalim. We can therefore assume that the refilling of both Ca2+ compartments sensitive to noradrenaline was inhibited.
In conclusion, these results are consistent with the contraction of the rat aorta in response to noradrenaline in Ca2+-free medium consisting of an initial phasic response and a tonic one. The former is due to the release of internal Ca2+ from a compartment refilled through a special channel that is cromakalim but not dihydropyridine sensitive. The tonic response is due to Ca2+ release from another compartment refilled through a cromakalim- and dihydropyridine-sensitive Ca2+ channel. The Ca2+ entry through this latter channel intervenes in the IRT observed during the refilling of these stores previously depleted by noradrenaline, and the opening state of this channel is also modulated by ATP-dependent K2+ channels.
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