Cyclopiazonic acid and thapsigargin reduce Ca2+ influx in frog skeletal muscle fibres as a result of Ca2+ store depletion

Stochasticity intrinsic to coupled-clock mechanisms underlies beat-to-beat variability of spontaneous action potential firing in sinoatrial node pacemaker cells

Recent evidence indicates that the spontaneous action potential (AP) of isolated sinoatrial node cells (SANCs) is regulated by a system of stochastic mechanisms embodied within two clocks: ryanodine receptors of the "Ca(2+) clock" within the sarcoplasmic reticulum, spontaneously activate during diastole and discharge local Ca(2+) releases (LCRs) beneath the cell surface membrane; clock crosstalk occurs as LCRs activate an inward Na(+)/Ca(2+) exchanger current (INCX), which together with If and decay of K(+) channels prompts the "M clock," the ensemble of sarcolemmal-electrogenic molecules, to generate APs. Prolongation of the average LCR period accompanies prolongation of the average AP beating interval (BI). Moreover, the prolongation of the average AP BI accompanies increased AP BI variability. We hypothesized that both the average AP BI and AP BI variability are dependent upon stochasticity of clock mechanisms reported by the variability of LCR period. We perturbed the coupled-clock system by directly inhibiting the M clock by ivabradine (IVA) or the Ca(2+) clock by cyclopiazonic acid (CPA). When either clock is perturbed by IVA (3, 10 and 30 μM), which has no direct effect on Ca(2+) cycling, or CPA (0.5 and 5 μM), which has no direct effect on the M clock ion channels, the clock system failed to achieve the basal AP BI and both AP BI and AP BI variability increased. The changes in average LCR period and its variability in response to perturbations of the coupled-clock system were correlated with changes in AP beating interval and AP beating interval variability. We conclude that the stochasticity within the coupled-clock system affects and is affected by the AP BI firing rate and rhythm via modulation of the effectiveness of clock coupling.

Barium plateau potentials of CA1 pyramidal neurons elicit all-or-none extracellular alkaline shifts via the plasma membrane calcium ATPase

In many brain regions, synchronous neural activity causes a rapid rise in extracellular pH. In the CA1 region of hippocampus, this population alkaline transient (PAT) enhances responses from postsynaptic, pH-sensitive N-methyl-d-aspartate (NMDA) receptors. Recently, we showed that the plasma membrane Ca(2+)-ATPase (PMCA), a ubiquitous transporter that exchanges internal Ca(2+) for external H(+), is largely responsible for the PAT. It has also been shown that a PAT can be generated after replacing extracellular Ca(2+) with Ba(2+). The cause of this PAT is unknown, however, because the ability of the mammalian PMCA to transport Ba(2+) is unclear. If the PMCA did not carry Ba(2+), a different alkalinizing source would have to be postulated. Here, we address this issue in mouse hippocampal slices, using concentric (high-speed, low-noise) pH microelectrodes. In Ba(2+)-containing, Ca(2+)-free artificial cerebrospinal fluid, a single antidromic shock to the alveus elicited a large (0.1-0.2 unit pH), "all-or-none" PAT in the CA1 cell body region. In whole cell current clamp of single CA1 pyramidal neurons, the same stimulus evoked a prolonged plateau potential that was similarly all-or-none. Using this plateau as the voltage command in other cells, we recorded Ba(2+)-dependent surface alkaline transients (SATs). The SATs were suppressed by adding 5 mM extracellular HEPES and abolished when carboxyeosin (a PMCA inhibitor) was in the patch pipette solution. These results suggest that the PAT evoked in the presence of Ba(2+) is caused by the PMCA and that this transporter is responsible for the PAT whether Ca(2+) or Ba(2+) is the charge carrying divalent cation.

Store-Operated Ca 2+ Influx and Expression of TRPC Genes in Mouse Sinoatrial Node

Store-operated Ca 2+ entry was investigated in isolated mouse sinoatrial nodes (SAN) dissected from right atria and loaded with Ca 2+ indicators. Incubation of the SAN in Ca 2+ -free solution caused a substantial decrease in resting intracellular Ca 2+ concentration ([Ca 2+ ] i ) and stopped pacemaker activity. Reintroduction of Ca 2+ in the presence of cyclopiazonic acid (CPA), a sarcoplasmic reticulum Ca 2+ pump inhibitor, led to sustained elevation of [Ca 2+ ] i , a characteristic of store-operated Ca 2+ channel (SOCC) activity. Two SOCC antagonists, Gd 3+ and SKF-96365, inhibited 72±8% and 65±8% of this Ca 2+ influx, respectively. SKF-96365 also reduced the spontaneous pacemaker rate to 27±4% of control in the presence of CPA. Because members of the transient receptor potential canonical (TRPC) gene family may encode SOCCs, we used RT-PCR to examine mRNA expression of the 7 known mammalian TRPC isoforms. Transcripts for TRPC1, 2, 3, 4, 6, and 7, but not TRPC5, were detected. Immunohistochemistry using anti-TRPC1, 3, 4, and 6 antibodies revealed positive labeling in the SAN region and single pacemaker cells. These results indicate that mouse SAN exhibits store-operated Ca 2+ activity which may be attributable to TRPC expression, and suggest that SOCCs may be involved in regulating pacemaker firing rate.

Changes in the functioning of the electromechanical connection during tetanic contraction

The functioning of the electromechanical connection during tetanic contraction in frog skeletal muscle was studied. Analysis using caffeine, calcium-free medium, the ryanodine receptor blocker dantrolene, and the Ca-ATPase inhibitor thapsigargin showed that the initial increase in tetanus, as in twitch contractions, did not require the presence of calcium ions in the surrounding medium, which is in agreement with published data. Contraction was accompanied by activation of potential-dependent release of calcium from the sarcoplasmic reticulum. In contrast, the secondary rise phase and/or the duration of the tetanus plateau were critically dependent on the present of Ca2+ in the surrounding medium. Given that contraction in this situation was inhibited by dantrolene, activation of prolonged contraction was also mediated by calcium released from the sarcoplasmic reticulum, though ryanodine receptors were now activated not by changes in the membrane potential but by the influx of external calcium. Thus, external calcium plays a significant role in the formation of prolonged contractile responses, providing for longer-lasting maintenance of power in contracted muscles.

Hormone-sensitive lipase activity and triacylglycerol hydrolysis are decreased in rat soleus muscle by cyclopiazonic acid

Cyclopiazonic acid (CPA) is a sarcoplasmic reticulum Ca2+-ATPase inhibitor that increases intracellular calcium. The role of CPA in regulating the oxidation and esterification of palmitate, the hydrolysis of intramuscular lipids, and the activation of hormone-sensitive lipase (HSL) was examined in isolated rat soleus muscles at rest. CPA (40 micro M) was added to the incubation medium to levels that resulted in subcontraction increases in muscle tension, and lipid metabolism was monitored using the previously described pulse-chase procedure. CPA did not alter the cellular energy state, as reflected by similar muscle contents of ATP, phosphocreatine, free AMP, and free ADP. CPA increased total palmitate uptake into soleus muscle (11%, P < 0.05) and was without effect on palmitate oxidation. This resulted in greater esterification of exogenous palmitate into the triacylglycerol (18%, P < 0.05) and phospholipid (89%, P < 0.05) pools. CPA decreased (P < 0.05) intramuscular lipid hydrolysis, and this occurred as a result of reduced HSL activity (20%, P < 0.05). Incubation of muscles with 3 mM caffeine, which is also known to increase Ca2+ without affecting the cellular energy state, reduced HSL activity (24%, P < 0.05). KN-93, a calcium/calmodulin-dependent kinase inhibitor (CaMKII), blocked the effects of CPA and caffeine, and HSL activity returned to preincubation values. The results of the present study demonstrate that CPA simultaneously decreases intramuscular triacylglycerol (IMTG) hydrolysis and promotes lipid storage in isolated, intact soleus muscle. The decreased IMTG hydrolysis is likely mediated by reduced HSL activity, possibly via the CaMKII pathway. These responses are not consistent with the increased hydrolysis and decreased esterification observed in contracting muscle when substrate availability and the hormonal milieu are tightly controlled. It is possible that more powerful signals or a higher [Ca2+] may override the lipid-storage effect of the CPA-mediated effects during muscular contractions.