Studies on the cellular mechanisms of action of positive and negative inotropic agents

Negative metabolic effects of cyclic GMP are altered in renal hypertension induced cardiac hypertrophy

We tested the hypothesis that increasing myocardial cyclic GMP levels would reduce myocardial O2 consumption and that renal hypertension (One Kidney-One Clip, 1K1C)-induced cardiac hypertrophy would change this relationship. Four groups of anesthetized open-chest New Zealand white rabbits (N = 26) were utilized. Either vehicle or 3-morpholinosydnonimine (SIN-1) (10(-4) M, a guanylate cyclase activator) was topically applied to the left ventricular surface of control or 1K1C rabbits. Coronary blood flow (radioactive microspheres) and O2 extraction (microspectrophotometry) were used to determine O2 consumption. Myocardial cyclic GMP levels were determined by radioimmunoassay. Guanylate cyclase activity was measured by conversion of GTP to cyclic GMP. 1K1C rabbits had a greater heart weight-to-body weight ratio (3.29 +/- 0.15) than controls (2.63 +/- 0.19). Systolic blood pressure was higher in 1K1C rabbits than in controls. In control rabbits, cyclic GMP levels (pmoles/g) were higher in SIN-1-treated (EPI: 7.5 +/- 1.6; ENDO: 8.1 +/- 1.5) than in vehicle-treated animals (EPI: 5.4 +/- 0.4; ENDO: 5.6 +/- 0.6). This effect was enhanced in 1K1C rabbits, with cyclic GMP levels in the SIN-1-treated group (EPI: 11.9 +/- 1.3; ENDO: 13.0 +/- 1.5) almost double those observed in the vehicle-treated group (EPI: 6.3 +/- 0.8; ENDO: 7.7 +/- 1.1). There were no significant differences in basal or maximally stimulated guanylate cyclase activity between controls and 1K1C rabbits. Myocardial O2 consumption (ml O2/min/100 g) was significantly less in the EPI region of control animals treated with SIN-1 (7.2 +/- 1.2) than in the same region of controls treated with vehicle (9.1 +/- 2.0). Myocardial O2 consumption was also significantly less in SIN-1-than vehicle-treated 1K1C animals (SIN-1-treated: EPI: 6.9 +/- 0.8; ENDO: 6.2 +/- 0.7; vehicle-treated: EPI: 10.0 +/- 0.8; ENDO: 12.5 +/- 3.0). There was no significant difference in O2 consumption between control and 1K1C animals after treatment with SIN-1. Thus, there was a greater elevation in cyclic GMP in 1K1C rabbits, but this did not result in a corresponding greater depression in O2 consumption. This suggests that cyclic GMP plays a role in the control of myocardial metabolism, and that the sensitivity of myocardial O2 consumption to changes in cyclic GMP is reduced by renal hypertension-induced cardiac hypertrophy.

Regulation of the calcium slow channel by cyclic GMP dependent protein kinase in chick heart cells

In order to assess the interaction between the cAMP-dependent and the cGMP-dependent phosphorylation pathways on the slow Ca2+ current (ICa(L)), whole-cell voltage-clamp experiments were conducted on embryonic chick heart cells. Addition of 8Br-cGMP to the bath solution reduced the basal (unstimulated) ICa(L). Intracellular application of the catalytic subunit of PK-A (PK-A(cat); 1.5 microM) via the patch pipette rapidly potentiated ICa(L) by 215 +/- 16%) (n = 4); subsequent addition of 1 mM 8Br-cGMP to the bath reduced the amplitude of ICa(L) towards the initial control values (123 +/- 29%). Intracellular application of PK-G (25 nM pre-activated by 10(-7) M cGMP), rapidly inhibited the basal ICa(L) by 64 +/- 6% (n = 8). Heat-denatured PK-G was ineffective. Subsequent additions of relatively high concentrations of 8Br-cAMP (1 mM) or isoproterenol (ISO, 1-10 microM) did not significantly remove the PK-G blockade of ICa(L). The results of the present study suggest that: (a) 8Br-cGMP can inhibit the basal or stimulated (by PK-A(cat)) ICa(L) in embryonic chick myocardial cells. (b) PK-G applied intracellularly inhibits the basal ICa(L).

Cyclic GMP regulation of calcium slow channels in cardiac muscle and vascular smooth muscle cells

The effects of cAMP and cGMP on the slow Ca2+ channels in cardiac muscle, VSM, and skeletal muscle fibers are summarized in Table V. As shown, in cardiac muscle, cAMP stimulates and cGMP inhibits. In VSM, both cAMP and cGMP inhibit. In skeletal muscle, both cAMP and cGMP stimulate.