Relationship between decreased function and O2 consumption caused by cyclic GMP in cardiac myocytes and L-type calcium channels

We tested the hypothesis that part of the decreased function and metabolism caused by cyclic guanosine monophosphate (GMP) in beating cardiac myocytes is related to inhibition of L-type calcium channels. The steady state oxygen consumption (VO₂) of a suspension of ventricular myocytes isolated from hearts of New Zealand white rabbits was measured using oxygen electrodes. Cellular cyclic GMP levels were determined by radioimmunoassay. Cell shortening was measured with a video edge detector. The VO₂ was obtained after: (1) adding sodium nitroprusside (NP 10^{?8,?6, ?4} M), (2) pretreatment by BAY K8644 10^?5 M (BAY, L-type calcium channel activator), nifedipine 10^?4 M (NF, L-type calcium channel blocker) or forskolin 10^?7 M (FK, adenylate cyclase activator), then adding NP10^?8,?6,?4 M, (3) pretreatment with both FK 10^?7 M and NF 10^?4 M and subsequently adding NP 10^{?8, ?6, ?4} M. NP 10^?4 M decreased VO₂ from 707±34 to 410±13 (nl O₂/min per 10⁵ myocytes), decreased the percentage of shortening (Pcs) from 5.7±0.6 to 3.7±0.5 and the rate of shortening (Rs) from 65.5±4.5 (µm/s) to 46.2±5.5. NP 10^?4 M also increased cyclic GMP from 264±70 (fmol/10⁵ myocytes) to 760±283. Both BAY and FK increased VO₂, Pcs and Rs without changing cyclic GMP. NF decreased Pcs, Rs and VO₂. Similar metabolic and functional effects of NP were observed with pretreatment with these agents separately, compared to NP alone, and the elevation of cyclic GMP level was not different from the control group. With FK alone, NP 10^?4 M decreased VO₂ by 51%, Pcs by 44% and Rs by 39%. In the presence of both FK and NF, the negative effects of NP were diminished significantly. NP 10^?4 M decreased VO₂ by 37%, Pcs by 25% and Rs 20%. Thus, in beating cardiac myocytes, the negative metabolic and functional effects of cyclic GMP were related to inhibition on L-type calcium channels only when adenylate cyclase was stimulated.