Effects of halothane and diltiazem on L-type calcium currents in single smooth muscle cells from rabbit portal veins

Food restriction promotes downregulation of myocardial L-type Ca2+ channels

Food restriction (FR) has been shown to impair myocardial performance. However, the mechanisms behind these changes in myocardial function due to FR remain unknown. Since myocardial L-type Ca2+ channels may contribute to the cardiac dysfunction, we examined the influence of FR on L-type Ca2+ channels. Male 60-day-old Wistar rats were fed a control or a restricted diet (daily intake reduced to 50% of the amount of food consumed by the control group) for 90 days. Myocardial performance was evaluated in isolated left ventricular papillary muscles. The function of myocardial L-type Ca2+ channels was determined by using a pharmacological Ca2+ channel blocker, and changes in the number of channels were evaluated by mRNA and protein expression. FR decreased final body weights, as well as weights of the left and right ventricles. The Ca2+ channel blocker diltiazem promoted a higher blockade on developed tension in FR groups than in controls. The protein content of L-type Ca2+ channels was significantly diminished in FR rats, whereas the mRNA expression was similar between groups. These results suggest that the myocardial dysfunction observed in previous studies with FR animals could be caused by downregulation of L-type Ca2+ channels.

Coupling of c-Src to large conductance voltage- and Ca2+-activated K+ channels as a new mechanism of agonist-induced vasoconstriction

The voltage-dependent and Ca(2+)-activated K(+) channel (MaxiK, BK) and the cellular proto-oncogene pp60(c-Src) (c-Src) are abundant proteins in vascular smooth muscle. The role of MaxiK channels as a vasorelaxing force is well established, but their role in vasoconstriction is unclear. Because Src participates in regulating vasoconstriction, we investigated whether c-Src inhibits MaxiK as a mechanism for agonist-induced vasoconstriction. Functional experiments in human and rat show that inhibitors of Src (Lavendustin A, PP2) but not inactive compounds (Lavendustin B, PP3) induce a pronounced relaxation of coronary or aortic smooth muscle precontracted with 5-hydroxytriptamine, phenylephrine, or Angiotensin II. Iberiotoxin, a MaxiK blocker, antagonizes the relaxation induced by Lavendustin A or PP2, indicating that c-Src inhibits the Iberiotoxin-sensitive component, likely MaxiK channels. In agreement, coronary muscle MaxiK currents were enhanced by Lavendustin A. To investigate the molecular mechanism of c-Src action on MaxiK channels, we transiently expressed its alpha subunit, hSlo, with or without c-Src in HEK293T cells. The voltage sensitivity of hSlo was right-shifted by approximately 16 mV. hSlo inhibition by c-Src is due to channel direct phosphorylation because: (i) excised patches exposed to protein tyrosine phosphatase (CD45) resulted in a partial reversal of the inhibitory effect by approximately 10 mV, and (ii) immunoprecipitated hSlo channels were recognized by an anti-phosphotyrosine Ab. Furthermore, coexpression of hSlo and c-Src demonstrate a striking colocalization in HEK293T cells. We propose that MaxiK channels via direct c-Src-dependent phosphorylation play a significant role supporting vasoconstriction after activation of G protein-coupled receptors by vasoactive substances and neurotransmitters.

The effects of halothane on single human neuronal L-type calcium channels

We investigated halothane's effects on the function of L-type Ca2+ channels in a human neuronal cell line, SH-SY5Y, by using the cell-attached patch voltage clamp configuration and Ba2+ as the charge carrier. In multiple-channel patches, halothane decreased the peak and persistent Ba2+ currents, accelerated the rate of inactivation, and slowed the rate of activation. Single-channel analysis showed that halothane (0.14-1.26 mM) increased the latency time for the first channel opening, increased the lifetime of nonconducting events, increased the proportion of short-lived open events, decreased the lifetime of the two open populations, and increased the percentage of current traces without channel activity. All of the observed halothane effects contribute to the halothane-induced decrease in macroscopic Ba2+ currents. The halothane concentration producing 50% reduction (IC50) of the peak Ba2+ current was 0.80 mM (approximately 1.9 hypothetical minimum alveolar anesthetic concentration [H-MAC] at 28 degrees C) and of the persistent Ba2+ current was 0.69 mM (approximately 1.7 H-MAC). The halothane effects did not always occur together, and the Hill slope of 1.6 suggested the presence of more than one interaction site or of more than one population of L-type Ca2+ channels. Halothane reduces L-type Ca2+ channel currents in human neuronal cells primarily through the stabilization of nonconducting states such as closed (before and after channel opening) and inactivated states.

IMPLICATIONS

Calcium is a signaling molecule in neurons. We measured the effect of halothane on Ba2+ (a Ca2+ surrogate) movement into a human neuron-like cell electronically. Ba2+ entry through the L-type channel was depressed. Halothane decreased the likelihood of the channel opening and enhanced the rate at which the channel closed and inactivated. These actions of halothane are probably related to its anesthetic action.