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We explored the effects of contractile arrest maintained for 24-72 h in the presence of 2,3-butanedione monoxime or a Ca(2+) channel blocker (nifedipine or verapamil) on contractile activity, Ca(i)(2+) transients, and myofibrillar protein content and ultrastructure in long-term cultures of spontaneously beating adult guinea-pig cardiomyocytes. The contractions were not affected by 5 mM 2, 3-butanedione monoxime, but they were strongly or fully suppressed by 10 and 18 mM 2,3-butanedione monoxime, respectively, while the Ca(i)(2+)transients triggered by the maintained spontaneous electrical activity were either not changed at all (5 and 10 mM 2, 3-butanedione monoxime) or decreased only slightly (18 mM 2, 3-butanedione monoxime). The uncoupling of excitation from contraction by 10-18 mM for 24-72 h did not affect the content of the myofibrillar proteins. Confocal laser microscopy showed that these exposures affected the assembly of myofilaments, giving an overall deranged appearance to the myofibrils. In spite of this effect, the cells' contractile activity was readily regained within 15-60 min upon the washout of 2,3-butanedione monoxime. The 24-72-h exposures to 5 microM nifedipine or 10 microM verapamil, which blocked fully both the Ca(i)(2+) transients and contractility, did not affect the myofibrillar protein content nor their assembly. However, the recovery of contractile activity after exposure to a Ca(2+)-channel blocker was significantly slower (several days) than after 2,3-butanedione monoxime exposure. Furthermore, cultures exposed to Ca(2+)-channel blockers also had significantly decreased sensitivity to beta-adrenergic stimulation. Altogether, these data indicate the importance of regular Ca(2+) influx for the maintenance of the functional integrity of adult cardiomyocytes during prolonged periods of contractile arrest.

We have examined the effects of R-phenylisopropyladenosine (R-PIA) and other adenosine analogues on basal, prostaglandin E1 (PGE1)- and forskolin-stimulated cyclic AMP (cAMP) formation in intact N1E-115 neuroblastoma cells, to determine whether the cells contain A1 adenosine receptors that are negatively coupled with adenylate cyclase. Basal levels of cAMP (68 +/- 7 pmol/mg protein; mean +/- SE, N = 15) were not altered by low concentrations of R-PIA. The apparent lack of inhibition was not due to increases in cAMP due to activation of a stimulatory A2 receptor by endogenously-synthesized adenosine. By comparison, low levels of R-PIA did reduce significantly (P less than 0.05) PGE1-dependent increases in cAMP formation (maximum response to PGE1, 972 +/- 77 pmol cAMP/mg protein; EC50 for PGE1, 0.2 microM). Inhibition was dose dependent, and resulted in a 30-50% maximum reduction in production stimulated by PGE1. Nanomolar concentrations of R-PIA elicited half-maximal inhibition; the inhibitory response was blocked by 8-phenyltheophylline (8-PT). The order of potencies of several adenosine analogues in eliciting this response suggested that inhibition was mediated by an A1 adenosine receptor. Examination of the effects of R-PIA on forskolin-stimulated cAMP formation yielded several interesting findings. First, stimulation by the diterpene by itself was blocked by both adenosine deaminase (ADA) and 8-PT (40 and 25% inhibition respectively). Low concentrations of R-PIA (less than 10(-6) M) had no effect on forskolin-stimulated cAMP production. At higher levels (greater than or equal to 10(-6) M) the analogues acted synergistically with the diterpene, to yield cAMP levels that were up to 3-fold higher than the additive effect of the two agents. Potentiation was stereospecific, Ca2+ dependent, and was blocked by 8-PT. The results of this study suggest that, in N1E-115 neuroblastoma cells, inhibitory A1 receptors are not stimulated in response to non-specific elevations in cAMP, but are associated with specific stimulatory receptors such as those activated by PGE1.