The effect of alteration of extracellular Na+ or Ca2+ and inhibition of Ca2+ entry, Na(+)-H+ exchange, and Na(+)-Ca2+ exchange by diltiazem, amiloride, and dichlorobenzamil on the response of cardiac cell aggregates to epidermal growth factor

EGF-induced sodium influx regulates EGFR trafficking through HDAC6 and tubulin acetylation

BACKGROUND

Endocytosis of activated EGF receptor (EGFR) to specific endocytic compartments is required to terminate EGF signaling. Trafficking of EGFR relies on microtubule tracks that transport the cargo vesicle to their intermediate and final destinations and can be modulated through posttranslational modification of tubulin including acetylation. Na,K-ATPase maintains intracellular sodium homeostasis, functions as a signaling scaffold and interacts with EGFR. Na,K-ATPase also binds to and is regulated by acetylated tubulin but whether there is a functional link between EGFR, Na,K-ATPase and tubulin acetylation is not known.

RESULTS

EGF-induced sodium influx regulates EGFR trafficking through increased microtubule acetylation. Increased sodium influx induced either by sodium ionophores or Na,K-ATPase blockade mimicked the EGF-induced effects on EGFR trafficking through histone deacetylase (HDAC) 6 inactivation and accumulation of acetylated tubulin. In turn, blocking sodium influx reduced tubulin acetylation and EGF-induced EGFR turnover. Knockdown of HDAC6 reversed the effect of sodium influx indicating that HDAC6 is necessary to modulate sodium-dependent tubulin acetylation.

CONCLUSIONS

These studies provide a novel regulatory mechanism to attenuate EGFR signaling in which EGF modulates EGFR trafficking through intracellular sodium-mediated HDAC6 inactivation and tubulin acetylation.

Indapamide accentuates cardiac chronotropic responses to epidermal growth factor in chick cardiomyocytes

We have previously shown that indapamide [chloro-4-N-(methyl-2-indolinyl-1)-sulfamoyal-3-benzamide] has a direct action on the heart to alter ion fluxes. This study sought to examine the potential interaction between indapamide and epidermal growth factor (EGF). Cardiomyocytes were prepared as primary culture from 7-day-old chick embryo hearts as aggregates that have a pattern of consistent spontaneous contraction. Indapamide enhanced the positive chronotropic response to EGF observed in chick embryonic ventricular myocyte aggregates while indapamide itself did not alter cardiac contractile frequency. Taken in conjunction with data that calcium channel blockade, inhibition of sodium entry or Na(+)-Ca2+ exchange in the cardiomyocyte opposes the positive chronotropic action of EGF on the cardiomyocyte, this study has identified an agent, indapamide, that accentuates the cardiomyocyte response to EGF.

Nifedipine does not alter the increased cystolic free magnesium during inhibition of mitochondrial function in isolated cardiac myocytes

1. The objectives of this investigation were to determine the effect of inhibition of mitochondrial function on intracellular free Mg concentration, [Mg2+]i, in the heart and to determine whether the calcium channel antagonist nifedipine would alter the response. 2. Cardiac myocytes were prepared as primary cultures from 7-day-old chick embryonic hearts. [Mg2+]i was determined in single ventricular cells with mag-fura-2. 3. Inhibition of mitochondrial function with carbonyl cyanide m-chlorophenylhydrozone (CCCP, 3 microM, plus amobarbital, 3 mM, produced a cessation of cardiac contractile frequency that was reversible. This was associated with an increase in [Mg2+]i from 0.48 to 0.98 mM which returned to near basal levels with removal of the drugs. [Ca2+]i oscillations with cell contraction were diminished in the presence of CCCP plus amobarbital and returned to normal following their removal. 4. In contrast, CCCP plus iodoacetate led to increased [Mg2+]i beyond 2 mM which was associated with elevated [Ca2+]i and cell death. 5. Nifedipine did not alter the cardiac contractile response to CCCP plus amobarbital. The increment in [Mg2+]i produced by CCCP plus amobarbital was not altered by nifedipine. These data suggest that [Mg2+]i is regulated by mitochondrial metabolism and nifedipine does not alter the increase in [Mg2+]i produced by inhibition of mitochondrial function suggesting increments in [Mg2+]i were from internal sources. Nifedipine may not interfere with the potentially beneficial actions of increased [Mg2+]i.

Indapamide alters the cyclic AMP signal transduction pathway in cardiomyocytes in culture

The cellular mechanism of action on the heart of the antihypertensive/diuretic agent indapamide is uncertain. Because of the importance of cAMP in signal transduction in the heart, the present study was designed to examine the hypothesis that indapamide interacts with the cAMP signal transduction pathway. Ventricular cardiomyocytes were isolated from 7-day-old chick embryo hearts and were maintained in culture. Indapamide did not alter basal and isoproterenol-stimulated cAMP levels in cardiomyocytes. However, indapamide pretreatment markedly accentuated forskolin-stimulated cAMP production. It also accentuated cholera toxin-induced increases in cAMP. Thus these data indicate that indapamide has an effect directly on the heart as manifested by its action on the ventricular cardiomyocytes. An effect of indapamide on cAMP generation by cholera toxin and forskolin suggests both a direct effect on adenylyl cyclase as well as an action through Gs. These findings may provide a cellular mechanism of action explaining indapamide's effect on the heart.

Lisinopril increases the recovery during reoxygenation and resistance to oxidative damage in cardiomyocytes

The action of the angiotensin-converting enzyme (ACE) inhibitor lisinopril on the consequences of myocardial reoxygenation and oxidative damage was assessed in cultured chick embryonic ventricular cardiomyocytes. Lisinopril, 10(-8) M to 10(-6) M, produced a significant (P < 0.05) dose-dependent enhancement of the restoration of contractile frequency occurring during myocardial reoxygenation but did not alter the depression in contractile frequency during hypoxia. Lisinopril significantly (P < 0.05) shifted the dose-response relationship of ammonium persulfate-induced reduction in cardiac contractile frequency. Lisinopril significantly (P < 0.05) reduced the effect of another oxidative agent, tertbutylhydroperoxide which produced a time-dependent reduction in cardiac contractile frequency. Lisinopril did not alter cardiac contractile frequency in the absence of hypoxia or ammonium persulfate or tertbutylhydroperoxide. The viability of cardiomyocytes, assessed by trypan blue exclusion, paralleled the changes in cardiac contractile frequency. Lisinopril significantly (P < 0.05) improved viability of cardiomyocytes exposed to either ammonium persulfate or tertbutylhydroperoxide. Lisinopril did not display any antioxidant properties against the free radical alpha,alpha-diphenyl-beta-picrylhydrazyl. These data suggest that lisinopril accelerates the recovery of cardiomyocytes during reoxygenation and blunts the effects of oxidative agents through mechanisms involving the endogenous renin angiotensin system and/or a direct cellular action.

Comparison of indapamide and hydrochlorothiazide on spontaneous contraction of cardiomyocytes in culture: the effect on alterations of extracellular calcium or potassium

1. The purpose of this study was to determine whether indapamide or hydrochlorothiazide had a direct action on the heart, specifically to determine whether either diuretic affected spontaneous contractile function of cardiomyocytes in culture or altered the response to increases in extracellular potassium or calcium. 2. Chick embryonic ventricular cells were cultured from 7-day-old chick embryo and myocardial cell aggregates were prepared. Spontaneous cardiac contractile frequency of ventricular myocyte aggregates was monitored. Indapamide [chloro-4-N-(methyl-2-indolinyl-1)-sulfamoyal-3-benzamide] over a concentration range of 10(-9)-10(-6) M did not alter cardiac contractile frequency. 3. Indapamide, in a dose dependent manner, significantly (P < 0.05) antagonized the effect of increases in extracellular potassium [K+]0 that produced a concentration dependent reduction in cardiac contractile frequency. In contrast, hydrochlorothiazide accentuated the effect of increased [K+]0 while hydrochlorothiazide did not alter spontaneous contractile frequency at the usual [K+]0 of 2.0 mM. 4. Indapamide produced a significant (P < 0.05) reduction in the effect of increases in extracellular calcium [Ca2+]0 on cardiac contractility while hydrochlorothiazide was associated with a slight accentuation of the effect of increased [Ca2+]0. Indapamide also slightly reduced the effect of the calcium agonist Bay K 8644, which increases intracellular calcium through voltage operated calcium channels. 5. These data indicate that diuretics modulate a cardiac response to changes in extracellular potassium and calcium. Indapamide antagonizes the effects of increases in extracellular potassium and calcium while hydrochlorothiazide has the opposite effect on the cardiac response to increases in [K+]0 or [Ca2+]0.