Heart myocytes as models of the cellular response to ischemia

Resting Ca2+ influx does not contribute to anoxia-induced cell death in adult rat cardiac myocytes

Calcium has been proposed as a primary influence on cell death during ischemic episodes in myocardial cells. One component of calcium entry into a cell is resting calcium influx. This basal movement of calcium is blocked by 100 micromol/L gadolinium chloride (GdCl3) in cardiac myocytes. Therefore, GdCl3 should be cardioprotective under anoxic conditions. To test this, cardiac myocytes isolated from adult male rats were subjected to anoxia (100% N2) in the presence or absence of 100 micromol/L GdCl3 in one of 2 ways. In the first method, cells were suspended in media and rendered anoxic for 0, 30, and 60 min, after which cell morphology and viability were scored. After 60 min of anoxia, rod-shaped cells accounted for 46% +/- 4% of total cells (viability 81%); 10 min of reoxygenation markedly reduced rod-shaped cells to 27% (viability 72%). GdCl3 in the medium did not protect the cells (anoxic rods 49%, reoxygenated rods 30%, viability 77%). In the second method, cells were attached to a laminin substrate, rendered anoxic, and then videotaped for up to 6 h. In this system, cells maintained their shape for some time after the onset of anoxia, and then began to 'die' (i.e., to take on either a rigor form or hypercontracted form) at a measurable rate. Time to onset of 'death' (t0), time to 50% and 100% 'death' (t50 and t100), and rate of 'death' were used to measure anoxic damage. Without GdCl3, cells on average began to die 115 +/- 32 min after the onset of anoxia (t0); they died at an average rate of 0.046 cells/min. t50 was achieved in 149 +/- 42 min, t100 in 183 +/- 54 min. Addition of 100 micromol/L GdCl3 did not affect any of these parameters. We concluded that GdCl3 was not cardioprotective for anoxic myocytes and that cell damage generated by anoxia could not be attributed to resting calcium influx.

3-isobutyl-1-methylxanthine (IBMX) sensitizes cardiac myocytes to anoxia

Cardiac myocytes incubated with 3-isobutyl-1-methylxanthine (IBMX), a nonspecific cyclic nucleotide phosphodiesterase inhibitor, formed rigor complexes under anoxic conditions more readily than cells incubated with other phosphodiesterase inhibitors. Cardiac myocytes were incubated for 1 hr with either (a) no additions, (b) 150 microM zaprinast, or (c) 1 mM IBMX, and then were rendered anoxic for periods up to 60 min. Cells were >80% viable throughout the anoxic period; viability was unaffected by either drug. Rod count decreased more rapidly after the onset of anoxia in the IBMX-treated cells than in control or zaprinast-treated cells (11% rods vs. roughly 47% rods after 30 min of anoxia). IBMX-treated cell groups also formed more "contracted" myocytes (box-like rods) than their untreated or zaprinast-treated counterparts (50% contracted vs. roughly 27% contracted after 30 min of anoxia). While nucleotide degradation patterns were similar in all experimental groups, the ratio of ATP to ADP was lower in IBMX-treated cells than in control or zaprinast-treated cells. The L-type calcium channel was apparently not involved in this phenomenon; while cyclic AMP was elevated in the IBMX-incubated cells, verapamil did not protect IBMX-incubated cells from premature damage by anoxia. Incubation with 8-cyclopentyl-1,3-dipropylxanthine (CDPX), an A1 receptor antagonist, at concentrations up to 1 microM in place of 1mM IBMX did not reproduce the IBMX effect. We concluded that IBMX sensitizes cardiac myocytes to anoxia through a mechanism related to its effect on ATP/ADP, and unrelated to an elevation of intracellular calcium or preconditioning phenomena.

Isolation and cultivation of Ca2+ tolerant cardiomyocytes from the adult rat: improvements and applications

1. Primary cultures of cardiomyocytes provide a valuable tool for the study of the pharmacological and toxicological properties of drugs and chemicals, but for several technical reasons cardiomyocytes from adult animals are not routinely used in long-term culture. Because of significant advances in cardiovascular research, tissue engineering and cell transplantation, the need to isolate primary cells from adult animal and/or human tissue is likely to increase in the future. 2. The most common protocols for the isolation and cultivation of cardiomyocytes have been reviewed and the various approaches have been compared. The recent advances in cell culture techniques and the use of the cytoprotective agent, e.g. 2,3-butanedione monoxime greatly increases cell yield and cell viability of isolated and cultured cardiomyocytes. New concepts emerge that enabled an assessment of cellular differentiation in cultured cardiomyocytes and certain specific nuclear transcription factors may play a pivotal role in this process.

Anoxia inhibits guanosine salvage in cardiac myocytes

The adult heart depends largely on salvage synthesis to supply its 5'-nucleotide needs. Previous work from this laboratory established that guanosine is metabolized into guanine 5'-nucleotides in heart cells, but that salvage rates are very slow as compared to adenosine. The author hypothesized that guanosine salvage is regulated according to the needs of the cell for guanine nucleotides. This hypothesis was tested using cardiac myocytes which were rendered anoxic for 0-60 min. During this anoxic period, guanine nucleotides were depleted about 50%. At 0, 30, and 60 min, aliquots were removed for cell counting and nucleotide analysis; 50 microM 3H-guanosine was then added and the incubation continued for 1 min. The cells were then extracted and assayed for radioactivity in the guanine nucleotide products. Anoxia for 60 min, depressed GTP levels by 89%, total guanine nucleotides by 50%, and short-term guanosine salvage by 48% over aerobic controls. Reoxygenation of the myocytes after 30 min of anoxia returned guanosine salvage rates to nearly normal (87% of control). Preincubation of the myocytes with 5 mM ribose for times up to 1 hour modestly increased salvage rates of guanosine in aerobic cells. These results suggest that guanosine salvage in cardiac myocytes is not regulated by the size of the guanine nucleotide pool (that is, not sensitive to the demand for guanine nucleotides). Instead, salvage of guanosine is probably limited by cytosolic levels of ATP or phosphoribosylpyrophosphate, the production of which are dependent on adequate oxygen supplies.