Effect of low-density lipoproteins, mevinolin, and G proteins on Ca2+ response in cultured chick atrial cells.

Growth of cells from atria of embryonic chick hearts 14 days in ovo in medium supplemented with lipoprotein-depleted serum (LPDS) results in an increase in total cell cholesterol, enhanced parasympathetic responsiveness (7), and decreased sympathetic responsiveness (1). These effects were reversed by the hydroxymethyl glutaryl CoA reductase inhibitor, mevinolin. In these studies, comparison of cell growth in medium supplemented with fetal calf serum (FCS) and LPDS demonstrated that, after growth with LPDS, the ability of Ca2+ and the Ca2+ channel agonist, BAY K 8644, to enhance the amplitude of contraction decreased by 25 and 50%, respectively. These effects of growth in LPDS were reversed by incubation with mevinolin. LPDS had no effect on either Ca2+ channel number as measured by (+)-[5-methyl-3H]PN200-110 binding or Ca2+ current density as measured by the whole cell patch method. Treatment of cells grown in LPDS with pertussis toxin, which inactivates alpha o and alpha i, returned the contractile response to 10(-7) M BAY K 8644 to control levels. Pertussis toxin had no effect on the contractile response or adenosine 3',5'-cyclic monophosphate levels in control cells grown in FCS alone. These data suggest that alterations in the relative levels of alpha o and alpha s in cells grown in LPDS may play a role in regulating the contractile response to Ca2+ channel agonists and to exogenous Ca2+.

Effect of manganese (II) bis(glycinate)dichloride on Ca2+ channel function in cultured chick atrial cells.

Manganese (II) bis(glycinate)dichloride (Mn(glycinate)2) is a coordination complex of manganese with application as a contrast enhancement agent for magnetic resonance imaging in the heart. To determine the cardioactivity of the manganese ion in this chelation cage, the effects of Mn(glycinate)2 on Ca channel function in the cultured chick atrial cell was studied. Mn(glycinate)2 decreased amplitude of contraction in chick atrial cells from embryos 14 days in ovo with complete inhibition of beating at 1 mM and half-maximal effect at 0.1 mM. Under control conditions, Bay K 8644, a Ca channel activator increased amplitude of contraction by 86% with a half maximal effect at 3.2 x 10(-7) M. In the presence of 0.025 mM Mn(glycinate)2, a concentration which had no effect on the amplitude of contraction, the maximum response to Bay K 8644 was decreased to 31%. Mn(glycinate)2 had no effect on the EC50 for the response to Bay K 8644, 1.7 +/- 0.1 x 10(-9) M (S.E.M., n = 4) in control cells compared to 2.2 +/- 0.4 x 10(-9) M (S.E.M., n = 4) in cells incubated with Mn(glycinate)2. 45Ca2+ uptake over 5 min in cultured chick atrial cells decreased from 2.0 nmol/mg protein in control cells to 1.5 nmol/mg protein in the presence of 10(-5) M PN200-110, a Ca2+ channel blocker, a decrease of 28%. 45Ca2+ uptake decreased to 0.94 nmol/mg protein (53%) in the presence of 1 nmol Mn(glycinate)2. Effects of Mn(glycinate)2 and PN200 were not additive. These data demonstrate that Mn(glycinate)2 exerts its negative inotropic effect, at least partially, by interfering with the function of the L-type Ca channels at high concentrations.