ZhuGe R, Li S, Chen T H, Hsu W H
Department of Veterinary Physiology and Pharmacology, Iowa State University, Ames 50011, USA.
Biol Reprod. 1997 May;56(5):1343-50. doi: 10.1095/biolreprod56.5.1343.
We studied the mechanisms underlying alpha2-adrenergic receptor (AR)-mediated increase in intracellular free calcium ([Ca2+]i) in freshly dispersed myometrial cells from sows in the luteal phase of the estrous cycle. After the blockade of beta-ARs with propranolol, epinephrine increased [Ca2+]i dose-dependently in both the presence and absence of extracellular Ca2+. The rank order of alpha antagonists in inhibiting [Ca2+]i response to epinephrine was yohimbine > WB4101 >> prazosin in both the presence and absence of extracellular Ca2+, suggesting that epinephrine acts on alpha(2A)-ARs to increase Ca2+ influx as well as Ca2+ release from intracellular stores. Thapsigargin, the blocker of the Ca2+ pump in the sarcoplasmic reticulum, abolished the release but did not affect the influx. Pertussis toxin (PTX) inhibited the influx but failed to change the release. Nimodipine, an L-type Ca2+ channel blocker, nearly abolished the influx. The peak increase in [Ca2+]i caused by epinephrine was reached within 20 sec of administration. Intracellular cAMP concentrations were also decreased at 20 sec post-epinephrine. Epinephrine enhanced the L-type Ca2+ channel current, whereas forskolin suppressed it. Maximization of intracellular cAMP content by applying 8-bromo-cAMP (100 microM) blocked the effect of epinephrine on the current. U-73122, a phospholipase C inhibitor, reduced the Ca2+ release by epinephrine and oxytocin. Our results suggested that 1) activation of alpha2-ARs induces Ca2+ influx through opening L-type Ca2+ channels as well as inducing Ca2+ release from intracellular stores, and 2) a PTX-sensitive G protein couples negatively to adenylyl cyclase, leading to a decrease in cAMP formation which may be involved in the activation of Ca2+ channels. In addition, our results are consistent with the coupling of alpha2-ARs to a PTX-insensitive G protein (G(q)) to release Ca2+ from intracellular stores.
我们研究了发情周期黄体期母猪新鲜分离的子宫肌层细胞中α2 - 肾上腺素能受体（AR）介导的细胞内游离钙（[Ca2+]i）增加的潜在机制。用普萘洛尔阻断β - ARs后，无论细胞外有无Ca2+，肾上腺素均能剂量依赖性地增加[Ca2+]i。在细胞外有无Ca2+的情况下，α拮抗剂抑制[Ca2+]i对肾上腺素反应的强度顺序均为育亨宾>WB4101>>哌唑嗪，这表明肾上腺素作用于α(2A)-ARs以增加Ca2+内流以及从细胞内储存库释放Ca2+。毒胡萝卜素是肌浆网中Ca2+泵的阻滞剂，它消除了Ca2+释放但不影响内流。百日咳毒素（PTX）抑制内流但未能改变释放。尼莫地平是一种L型Ca2+通道阻滞剂，几乎消除了内流。肾上腺素给药后20秒内达到[Ca2+]i的峰值增加。肾上腺素作用后20秒细胞内cAMP浓度也降低。肾上腺素增强L型Ca2+通道电流，而福斯可林抑制该电流。应用8 - 溴 - cAMP（100 microM）使细胞内cAMP含量最大化可阻断肾上腺素对电流的作用。磷脂酶C抑制剂U - 73122减少了肾上腺素和催产素引起的Ca2+释放。我们的结果表明：1）α2 - ARs的激活通过打开L型Ca2+通道诱导Ca2+内流以及诱导从细胞内储存库释放Ca2+；2）一种对PTX敏感的G蛋白与腺苷酸环化酶负偶联，导致cAMP形成减少，这可能参与Ca2+通道的激活。此外，我们的结果与α2 - ARs与一种对PTX不敏感的G蛋白（G(q)）偶联以从细胞内储存库释放Ca(2+)一致。
