Impaired alpha1-adrenergic responses in aged rat hearts.

To determine age-related changes in the cardiac effect of alpha1-adrenergic stimulation, both cardiomyocyte Ca2+-transient and cardiac protein kinase C (PKC) activity were measured in 3-month- (3MO) and 24-month- (24MO) old Wistar rats. Ca2+ transients obtained under 1 Hz pacing by microfluorimetry of cardiomyocyte loaded with indo-1 (405/480 nm fluorescence ratio) were compared in control conditions (Kreb's solution alone) and after alpha1-adrenergic stimulation (phenylephrine or cirazoline, an alpha1-specific agonist). PKC activity and PKC translocation index (particulate/total activity) were also assayed before and after alpha1-adrenergic stimulation. In 3MO, cirazoline induced a significant increase in Ca2+ transient for a 10(-9) M concentration which returned to control values for larger concentrations. In contrast, in 24MO, we observed a constant negative effect of cirazoline on the Ca2+ transient with a significant decrease at 10(-6) M compared with both baseline and Kreb's solution. Preliminary experiments showed that, in a dose-response curve to phenylephrine, the response of Ca2+ transient was maximal at 10(-7) M. This concentration induced a significant increase in Ca2+ transient in 3MO and a significant decrease in 24MO. The same concentration was chosen to perform PKC activity measurements under alpha1-adrenergic stimulation. In the basal state, PKC particulate activity was higher in 24MO than that in 3MO but was not different in cytosolic fractions; so that the translocation index was higher in 24MO (P < 0.01). After phenylephrine, a translocation of PKC toward the particulate fraction was observed in 3MO but not in 24MO. In conclusion, cardiac alpha1-adrenoceptor response was found to be impaired in aged hearts. The negative effect of alpha1-adrenergic stimulation on Ca2+ transient in cardiomyocytes obtained from old rats can be related to an absence of alpha1-adrenergic-induced PKC translocation.

Interaction of chelerythrine with inositol phosphate metabolism.

Chelerythrine, a potent inhibitor of protein kinase C (PKC), was evaluated for its effect on inositol phosphate (IP) metabolism in newborn rat cardiomyocytes in culture. In a first step, we evaluated the effect of chelerythrine on IP accumulation in basal conditions. For a 10(-4) M dose, 5-phosphatase activity (which dephosphorylates IP3 into IP2) was completely blocked and we observed a large increase in IP accumulation limited to IP2 without any increase in IP3, strongly suggesting that chelerythrine at this dose modifies IP metabolism. At a lower dose (10(-5) M) of chelerythrine, which did not modify IP accumulation and 5-phosphatase activity in basal conditions, the response to angiotensin II stimulation was completely abolished by the addition of chelerythrine. We conclude thus that chelerythrine, even at 10(-5) M, interacts markedly with IP metabolism, and caution should be exerted when interpreting the results obtained with this drug, which is still currently used at this dose.