Possible mechanism underlying the potent vasoconstrictor actions of cyclopiazonic acid on dog cerebral arteries

Unrepeatable extracellular Ca2+-dependent contractile effects of cyclopiazonic acid in rat vascular smooth muscle

Cyclopiazonic acid (CPA), a specific reversible inhibitor of Ca(2+)-pumps in sarcoplasmic reticulum, causes a slowly developing and subsequently diminishing characteristic contraction in endothelium-denuded rat vascular smooth muscle. We recently found that CPA-induced contractions were not completely repeatable in endothelium-denuded rat aorta and superior mesenteric artery. 10 microM CPA-induced contractions expressed as a percentage of 80 mM KCl-induced contraction were significantly decreased from 51.4+/-5.7% to 11.8+/-2.6% (P<0.0001) upon the second application in endothelium-denuded rat aorta, and this was not due to any irreversible cytotoxic effects of CPA. The decrease of CPA-induced contractile responses upon the second application was dependent on both types of blood vessels and doses of CPA upon the first application. CPA upon the second application in Ca(2+)-containing solutions did induce its characteristic contractions in the rings pretreated with Ca(2+)-free solutions or Ca(2+) entry blockers before and during its first application, suggesting that capacitative mode of Ca(2+) influx during the application of CPA might be responsible for the diminishment of contractions upon the second application. These data suggest that CPA by inducing a transient rise in cytosolic Ca(2+) level might cause a long-lasting upregulation of Ca(2+) extrusion across the plasma membrane in vascular smooth muscle cells and thus accelerate Ca(2+) efflux over a prolonged period, leading to unrepeatable contractile effects of CPA. Such long-lasting upregulation of Ca(2+) extrusion may contribute to the regulation of excitability of vascular smooth muscle cells and protect the cells against excitotoxic injury.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Dependency of detrusor contractions on calcium sensitization and calcium entry through LOE-908-sensitive channels

1. The subcellular mechanisms regulating stimulus-contraction coupling in detrusor remain to be determined. We used Ca(2+)-free solutions, Ca(2+) channel blockers, cyclopiazonic acid (CPA), and RhoA kinase (ROK) inhibitors to test the hypothesis that Ca(2+) influx and Ca(2+) sensitization play primary roles. 2. In rabbit detrusor, peak bethanechol (BE)-induced force was inhibited 90% by incubation for 3 min in a Ca(2+)-free solution. By comparison, a 20 min incubation of rabbit femoral artery in a Ca(2+)-free solution reduced receptor-induced force by only 5%. 3. In detrusor, inhibition of sarcoplasmic reticular (SR) Ca(2+) release by 2APB, or depletion of SR Ca(2+) by CPA, inhibited BE-induced force by only 27%. The CPA-insensitive force was abolished by LaCl3. By comparison, 2APB inhibited receptor-induced force in rabbit femoral artery by 71%. 4. In the presence of the non-selective cation channel (NSCC) inhibitor, LOE-908, BE did not produce an increase in [Ca(2+)]i but did produce weak increases in myosin phosphorylation and force. 5. Inhibitors of ROK-induced Ca(2+) sensitization, HA-1077 and Y-27632, inhibited BE-induced force by approximately 50%, and in combination with LOE-908, nearly abolished force. 6. These data suggest that two principal muscarinic receptor-stimulated detrusor contractile mechanisms include NSCC activation, that elevates [Ca(2+)]i and ROK activation, that sensitizes cross bridges to Ca(2+).

Protein kinase C and cerebral vasospasm

Twenty-five years after the discovery of protein kinase C (PKC), the physiologic function of PKC, and especially its role in pathologic conditions, remains a subject of great interest with 30,000 studies published on these aspects. In the cerebral circulation, PKC plays a role in the regulation of myogenic tone by sensitization of myofilaments to calcium. Protein kinase C phosphorylates various ion channels including augmenting voltage-dependent Ca2+ channels and inhibiting K+ channels, which both lead to vessel contraction. These actions of PKC amplify vascular reactivity to different agonists and may be critical in the regulation of cerebral artery tone during vasospasm. Evidence accumulated during at least the last decade suggest that activation of PKC in cerebral vasospasm results in a delayed but prolonged contraction of major arteries after subarachnoid hemorrhage. Most of the experimental results in vitro or in animal models support the view that PKC is involved in cerebral vasospasm. Implication of PKC in cerebral vasospasm helps explain increased arterial narrowing at the signal transduction level and alters current perceptions that the pathophysiology is caused by a combination of multiple receptor activation, hemoglobin toxicity, and damaged neurogenic control. Activation of protein kinase C also interacts with other signaling pathways such as myosin light chain kinase, nitric oxide, intracellular Ca2+, protein tyrosine kinase, and its substrates such as mitogen-activated protein kinase. Even though identifying PKC revolutionized the understanding of cerebral vasospasm, clinical advances are hampered by the lack of clinical trials using selective PKC inhibitors.

Ca(2+) uptake function of sarcoplasmic reticulum during contraction of rat arterial smooth muscles

To determine the Ca(2+) uptake function of the sarcoplasmic reticulum during contraction, the effects of cyclopiazonic acid or thapsigargin, agents that inhibit sarcoplasmic reticulum Ca(2+)-ATPase, on the contractile responses to K(+) or norepinephrine were compared in endothelium-denuded strips of femoral, mesenteric and carotid arteries of the rat. The addition of K(+) (3-20 mM) to the strips caused a concentration-dependent contraction, and the sensitivity to K(+) was much higher in the carotid artery than in the other arteries. The preincubation of strips with cyclopiazonic acid (10 microM) or thapsigargin (100 nM) caused a leftward shift of the concentration-response curve for K(+), and this effect was smaller in the carotid artery than in the other arteries. Inhibition of sarcoplasmic reticulum Ca(2+) uptake caused the sensitivity to K(+) to be similar in the three arteries. Similar results were obtained when the contractile responses to norepinephrine were determined. Cyclopiazonic acid itself induced similar transient contractions in the three arteries. The addition of caffeine (20 mM) caused a transient contraction that was smaller in the carotid artery than in the other arteries. We conclude that (1) the Ca(2+) influx during stimulation with K(+) or norepinephrine is buffered by the sarcoplasmic reticulum in femoral and mesenteric arteries, (2) this function is weak in the carotid artery, probably because the sarcoplasmic reticulum of this artery is almost filled with Ca(2+) in the resting state, and (3) the Ca(2+) uptake function of the sarcoplasmic reticulum during contraction is reflected by the contractile sensitivity in these arteries.