A study of intracellular calcium oscillations in sheep cardiac Purkinje fibres measured at the single cell level.

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Previous work has shown that an elevation of intracellular calcium concentration [( Ca2+]i) produces spontaneous oscillations of [Ca2+]i. However the fact that the oscillations are unsynchronized between different cells has made it difficult to study them. We have therefore injected only one cell in a Purkinje fibre with aequorin in order to avoid these problems. The addition of strophanthidin (10 microM) produced an increase of mean aequorin light over the course of several minutes. During this period spontaneous oscillations of light developed and, with time, their frequency and magnitude increased. The oscillations could first be seen at levels of [Ca2+]i of less than 1 microM. The amplitude of the oscillations of [Ca2+]i could be up to 10 microM and was modulated at a slow rate (about 0.3-0.5 Hz). This suggests that, even within one cell, different regions may oscillate at different frequencies. Elevating [Ca+]o, removing extracellular Na+, or depolarization increased the magnitude of the aequorin light oscillations. Converting the records to [Ca2+]i showed that this increase in the magnitude of the aequorin oscillations was accompanied by a real increase of mean [Ca2+]i and of the magnitude of the oscillations [Ca2+]i. The frequency of the oscillations increased up to a point but saturated at a maximum value of 3-4 Hz. Since previous experiments have used the mean aequorin light to estimate mean [Ca2+]i, we have calculated the error produced in this calculation by the presence of [Ca2+]i oscillations. We estimate that the error is greatest at low levels of Ca2+ loading when the frequency of the oscillations is low. However, at higher Ca2+ loads, when the frequency is above 2 Hz, the error is probably less than 10%. If oscillations were produced by removal of external Na+ after the application of strophanthidin, then either ryanodine or caffeine abolished the oscillations. Furthermore, in both cases, the resulting steady level of [Ca2+]i was similar to the mean level before the addition of the drugs. In another series of experiments we examined the effects of these drugs on oscillations produced by the application of strophanthidin. Caffeine produced a transient increase in both the frequency of the oscillations and mean [Ca2+]i before abolishing the oscillations and decreasing [Ca2+]i to below the level in the absence of caffeine. In contrast ryanodine gradually decreased both the mean [Ca2+]i and the frequency until the oscillations were abolished. During this period of slowing of the oscillations their magnitude was often increased.

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