Figure 3.

Effect of nifedipine on IP₃ mass changes following K⁺ depolarization in primary muscle cultures. Confluent plates of myotubes were washed three times with PBS and incubated during the times indicated with 47 mM K⁺, both in the presence (B and C) and in the absence (A) of extracellular calcium. The mass of IP₃ in the extract was measured by radioreceptor assay. (A) Time course for the increase in the mass of IP₃ in the absence of external calcium. The biphasic nature (an early fast and a later slower component) of the increase is clearer here than in B because measurements were made at shorter time intervals (2.0 s) during the onset of the transient. Values were expressed as the mean ± SD of at least three different samples from the same experiment. Paired t-tests were performed (**, P < 0.0006 [n = 4]; ***, P < 0.0004 [n = 4]) comparing each point with value at t = 0. (B) Samples incubated in the absence (filled circles) or in the presence (open circles) of 10 μM nifedipine. Note the slow kinetics of the transient increase in IP₃ mass in the presence of nifedipine. Values were expressed as the mean ± SD of at least three different samples from a single experiment done in duplicate, for control and in the presence of nifedipine. Thus, these two curves can be directly compared. Paired t tests, comparing each point were performed (#, P < 0.0002 [n = 4] at t = 5 s; ##, P < 0.0001 [n = 4] at t = 15 s; @, P < 0.0005 [n = 3] at t = 30 s; &, P < 0.0047 [n = 3] at t = 45 s). (C) Mean values for control and depolarized cells after 15 s of high potassium exposure both in the absence and in the presence of 10 μM pertussis toxin. Error bars represent SD.