The Lack of Toxicity of Potassium ChannelActivators in Heart Cell Cultures.

High doses of the potassium channel activators (KCAs) BRL 44269, levcromakalin and pinacidil in a number of laboratory animal species cause a profound reduction in blood pressure which results in reflex tachycardia, ischaemia and myocardial necrosis. Thus, it is considered that the in vivo cardiac pathology seen with KCAs is an indirect effect as a consequence of excessive pharmacological effects rather than direct myocardial toxicity. This hypothesis was tested, in vitro, in the chick embryonic myocardial myocyte reaggregate (MMR) model system. Changes in spontaneous beating activity (SBA), leakage of lactate dehydrogenase (LDH) and cell morphology by light and transmission electron microscopy were used to assess toxicity. The MMRs were cultured for up to 24hr in a series of different concentrations of the three KCAs in the range 1-10,000mum. In addition to an untreated control, allylamine (50mum), a known direct acting myocardial cytotoxin, was used as a positive control. Incubation with allylamine caused clear evidence of toxicity and permanent cessation of SBA. In contrast, KCAs caused changes in SBA and significant toxicity was only seen at the highest concentration (10,000mum) of BRL 44269. These results are supportive of the view that KCA-induced cardiac pathology in vivo is due to an indirect pharmacological action rather than a direct, cytotoxic mechanism.

Study of the effects of cardiovascular drugs in heart cell cultures.

Compounds that produce myocardial pathology in vivo can be separated into two main classes-those that are directly toxic to the myocardium and those that are considered to act by way of an indirect vascular or neurologically based mechanism. An in vitro model of myocardium without nervous or systemic influences can be used to differentiate between direct myocardial cytotoxic effects and indirect cardiac pathology arising in vivo from exaggerated vascular or neural pharmacological effects of a number of drugs. In this study direct-acting cardiotoxic compounds are distinguished from those causing cardiac pathology by indirect mechanisms by their different pattern of effects in chick embryonic myocardial myocyte reaggregates (MMRs) cultures. The toxicity of the direct-acting cardiotoxic drugs allylamine (positive control, 50 mum) and doxorubicin were compared with digoxin and isoprenaline, which show both direct and indirect mechanisms in vivo, and the indirectly acting hydralazine and pinacidil. Changes in spontaneous beating activity (SBA), leakage of lactate dehydrogenase (LDH) and cell morphology by light and transmission electron microscopy were used to assess toxicity. The MMRs were cultured for up to 24 hr in a series of concentrations of the five compounds in the range 0.1 to 10,000 mum. Allylamine, doxorubicin, digoxin and, to a lesser extent, isoprenaline were highly toxic to the MMRs, as shown by alterations in SBA, LDH leakage and cellular morphology. In contrast, hydralazine showed a very mild degree of toxicity at the highest concentrations in the absence of LDH leakage; treatment with pinacidil did not show any evidence of morphological degeneration but did cause a dose-related inhibition of SBA. These results are consistent with the view that doxorubicin and digoxin are directly toxic to myocardial cells and also suggests that this is an important mechanism in vivo for isoprenaline. The absence of a significant degree of toxicity with hydralazine and pinacidil is consistent with an indirect toxic mechanism.

Study of the cardiotoxic potential of pharmaceutical compounds in chick myocardial myocyte reaggregate cultures.

Cardiotoxicity produced by doxorubicin in vivo is considered to be due to a direct effect on the myocardium and this is also a major component with toxicity of isoprenaline and digoxin. In the case of the cardiotoxicity produced at high doses by the antihypertensives hydralazine and pinacidil, an indirect mechanism operating by way of their exaggerated pharmacological effects is believed to be responsible. These compounds were examined for their cardiotoxic potential in vitro using chick myocardial myocyte reaggregate (MMR) cultures; allylamine HCl was used as a positive control. Cultures were incubated for up to 24 hr with each compound; parameters analysed were: spontaneous beating activity (SBA), lactate dehydrogenase (LDH) leakage and microscopic evidence of cytotoxicity. Allylamine, doxorubicin, digoxin and to a lesser extent isoprenaline were highly toxic to MMR cultures, as demonstrated by their effects on SBA, LDH leakage and morphology. Hydralazine showed very mild cytotoxicity at the highest concentrations with no LDH leakage; pinacidil was not cytotoxic but showed a dose-related inhibition of SBA. These results confirm the direct toxic action of doxorubicin and digoxin on myocardial cells and indicate that this is also an important mechanism in vivo for isoprenaline. The lack of any significant toxicity with hydralazine and pinacidil accords with an indirect mechanism based on their pharmacology.