Negative metabolic effects of cyclic GMP in quiescent cardiomyocytes are not related to L-type calcium channel activity

Abstract

We tested the hypothesis that the negative metabolic effects of elevating cyclic GMP act through inhibition of L-type calcium channels in quiescent cardiac myocytes. The steady state O₂ consumption (VO₂) of ventricular myocytes, isolated from hearts of New Zealand white rabbits, was measured in a glass chamber using Clark-type oxygen electrodes. The cellular cyclic GMP levels were determined by radioimmunoassay at baseline with either 0.5 mM or 2.0 mM of Ca²⁺, sodium nitroprusside at increasing concentration (10^{−8, −6, −4} M) with and without pretreatment by BAY K8644 10⁻⁵ M (L-type Ca²⁺ channel activator) in 0.5 mM Ca²⁺, or nitroprusside with and without pretreatment with nifedipine 10⁻⁴ M (L-type Ca²⁺ channel blocker) in 2.0 mM Ca²⁺. In the 0.5 mM Ca²⁺ medium, basal VO² was 459±104 (nl O²/min per 10⁵ myocytes) with a corresponding cyclic GMP level of 112±23 (fmol/10⁵ myocytes). With nitroprusside 10⁻⁴ M, VO₂ was decreased to 285±39 and cyclic GMP level was significantly elevated to 425±128. In the same medium, VO₂ was slightly increased by BAY K8644 10⁻⁵ M while the cyclic GMP level did not change. With BAY K8644 10⁻⁵ M, nitroprusside 10⁻⁴ M decreased VO₂ and increased cyclic GMP to a level which was similar to cells treated with nitroprusside alone. In the 2.0 mM Ca²⁺ medium, the basal VO² and cyclic GMP were 518±121 and 137±24. In the presence of nitroprusside 10⁻⁴ M, VO₂ was decreased to 295±49 and cyclic GMP was increased to 454±116. In the same medium, nifedipine 10⁻⁴ M significantly decreased VO₂, while the cyclic GMP level was comparable to the baseline. After nifedipine 10⁻⁴ M, nitroprusside 10⁻⁴ M decreased VO₂ and increased cyclic GMP to levels which were similar to control. Therefore, in quiescent cardiac myocytes, the negative metabolic effects associated with cyclic GMP were not primarily mediated through inhibition of L-type Ca²⁺ channels.