Cyclopiazonic acid alters serotonin-induced responses in rat thoracic aorta

Inhibitory Effects of Cyclopiazonic Acid on the Pacemaker Current in Sinoatrial Nodal Cells

OBJECTIVE

The spontaneous action potential of isolated sinoatrial node (SAN) cells is regulated by a coupled-clock system of two clocks: the calcium clock and membrane clock. However, it remains unclear whether calcium clock inhibitors have a direct effect on the membrane clock. The purpose of this study was to investigate the direct effect of cyclopiazonic acid (CPA), a selective calcium clock inhibitor, on the function of the membrane clock of SAN cells.

METHODS

at SAN cells were isolated by trypsinization and identified based on morphology and electrophysiology. I_f and HCN currents were recorded via patch clamp technique. The expression of the HCN channel protein was determined by Western blotting analysis.

RESULTS

The diastolic depolarization rate of spontaneous action potentials and the current densities of I_f were reduced by exposure to 10 μM CPA. The inhibitory effect of CPA was concentration-dependent with an IC₅₀ value of 16.3 μM and a Hill coefficient of 0.98. The effect of CPA on I_f current was also time-dependent, and the I_f current amplitude was partially restored after washout. Furthermore, the steady-state activation curve of the I_f current was shifted to a negative potential, indicating that channel activation slowed down. Finally, the protein expression of HCN4 in HEK293 cells was markedly downregulated by CPA.

CONCLUSIONS

These results indicate that the direct inhibition effect of CPA on the I_f current in SAN cells is both concentration- and time-dependent. The underlying mechanisms may involve slowing down steady-state activation and the downregulation of pacemaker channel protein expression.

Synchronized Astrocytic Ca2+ Responses in Neurovascular Coupling during Somatosensory Stimulation and for the Resting State

The role of astrocytes in neurovascular coupling (NVC) is unclear. Here, we applied a multimodality imaging approach to concomitantly measure synchronized neuronal or astrocytic Ca²⁺ and hemodynamic changes in the mouse somatosensory cortex at rest and during sensory electrical stimulation. Strikingly, we found that low-frequency stimulation (0.3-1 Hz), which consistently evokes fast neuronal Ca²⁺ transients (6.0 ± 2.7 ms latency) that always precede vascular responses, does not always elicit astrocytic Ca²⁺ transients (313 ± 65 ms latency). However, the magnitude of the hemodynamic response is increased when astrocytic transients occur, suggesting a facilitatory role of astrocytes in NVC. High-frequency stimulation (5-10 Hz) consistently evokes a large, delayed astrocytic Ca²⁺ accumulation (3.48 ± 0.09 s latency) that is temporarily associated with vasoconstriction, suggesting a role for astrocytes in resetting NVC. At rest, neuronal, but not astrocytic, Ca²⁺ fluctuations correlate with hemodynamic low-frequency oscillations. Taken together, these results support a role for astrocytes in modulating, but not triggering, NVC.

Effects of cyclopiazonic acid and dexamethasone on serotonin-induced calcium responses in vascular smooth muscle cells

We previously observed that sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA) blockade by cyclopiazonic acid (CPA) significantly potentiates serotonin (5-hydroxytryptamine (5-HT))-induced vascular contractions. Furthermore, 5-HT receptor antagonist methysergide partially inhibited CPA-potentiated 5-HT contractions. In the present study, we further investigated whether SERCA inhibition potentiates 5-HT-induced Ca(2+) responses along with attenuating the receptor antagonism by store-operated Ca(2+) (SOC) entry and protein kinase C (PKC)-mediated mechanisms. The effects of dexamethasone that was previously shown to induce SOC entry and enhance 5-HT responses were also tested. For this purpose, intracellular Ca(2+) levels were monitored in A7r5 embryonic rat vascular smooth muscle cells by spectrofluorometry using the fluorescent indicator fura-2. The results showed that CPA, although not dexamethasone, significantly potentiated 5-HT-induced Ca(2+) elevations. Ketanserin partially decreased 5-HT-induced and CPA-potentiated Ca(2+) elevations whereas both PKC inhibitor D-sphingosine and SOC entry blocker 2-aminoethoxydiphenyl borate (2-APB) abolished the remaining responses. The data suggests that diminished antagonistic effect on 5-HT-induced Ca(2+) elevations in the presence of SERCA inhibition is induced by SOC entry and PKC activation.