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Relationship between decreased function and O2 consumption caused by cyclic GMP in cardiac myocytes and L-type
1Department of Physiology and Biophysics, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School, Piscataway 08854-5635, USA. hweiss@umdnj.edu
Abstract:
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 NP 10(-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 10(5) 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 (microns/s) to 46.2 +/- 5.5. NP 10(-4) M also increased cyclic GMP from 264 +/- 70 (fmol/10(5) 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.
Insights
Cyclic guanosine monophosphate (GMP) reduces cardiac myocyte function by inhibiting L-type calcium channels, but only when adenylate cyclase is activated. This study investigates the link between cyclic GMP, calcium channels, and cardiac metabolism.
Area of Science:
- Cardiology
- Cell Physiology
- Biochemistry
Background:
- Cyclic guanosine monophosphate (cGMP) is a key intracellular second messenger.
- Decreased cardiac myocyte function and metabolism are observed with elevated cGMP.
- The precise mechanisms linking cGMP to cardiac function are not fully understood.
Purpose of the Study:
- To test the hypothesis that cGMP-induced decreases in cardiac myocyte function and metabolism involve inhibition of L-type calcium channels.
- To elucidate the role of L-type calcium channels in mediating the effects of cGMP in cardiac myocytes.
Main Methods:
- Ventricular myocytes were isolated from New Zealand white rabbit hearts.
- Oxygen consumption (VO2), cell shortening (Pcs), and shortening rate (Rs) were measured.
- cGMP levels were quantified using radioimmunoassay.
- Experiments involved pretreatment with L-type calcium channel modulators (BAY K8644, nifedipine) and an adenylate cyclase activator (forskolin) before adding sodium nitroprusside (NP) to elevate cGMP.
Main Results:
- Sodium nitroprusside (NP) decreased VO2, Pcs, and Rs, while increasing cGMP levels.
- L-type calcium channel activator (BAY) and adenylate cyclase activator (FK) increased VO2, Pcs, and Rs.
- Nifedipine (NF) decreased Pcs, Rs, and VO2.
- The inhibitory effects of NP on cardiac function and metabolism were significantly diminished when adenylate cyclase was stimulated (FK) and L-type calcium channels were blocked (NF).
Conclusions:
- In beating cardiac myocytes, the negative metabolic and functional effects of cGMP are linked to the inhibition of L-type calcium channels.
- This inhibition is specifically observed under conditions where adenylate cyclase activity is stimulated.
- The findings highlight a complex interplay between cGMP, calcium signaling, and metabolic regulation in the heart.