Figure 6. Reversibility of the effect of Ca²⁺ ionophore on the expression of slow MyHCI mRNA.

Cells were cultured for 22 (lane 1) or 30 days (lane 3) without ionophore or from day 8 for a further 14 days with Ca²⁺ ionophore A23187 (4 × 10⁻⁷ M) (lane 2). Other cells were cultured from day 8 to 22 with ionophore and then after withdrawal of ionophore for a further 8 days (lane 4). Total RNA (20 μg) was isolated from control and ionophore treated cultures on the days indicated, fractionated on a 1.2 % agarose-formaldehyde gel, and transferred to nitrocellulose. The blots were hybridized with the 3′ terminal ³²P-labelled HinfI fragment of MyHCI cDNA (1 × 10⁶ c.p.m. ml⁻¹) or an rDNA probe from 18S rRNA (1 × 10⁶ c.p.m. ml⁻¹). The positions of 18S rRNA (1.9 kb) and 28S rRNA (4.8 kb) on the ethidium bromide-stained gel are indicated.