Abundance of heteronuclear and messenger RNA for internal Ca pump in stomach smooth muscle and myocardium

The sarcoplasmic reticulum Ca pump gene SERCA2 is alternatively spliced to express mRNA encoding the protein SERCA2a in heart and SERCA2b in stomach smooth muscle. The expression of SERCA2 protein in heart is 70 +/- 10 fold that in stomach smooth muscle. To understand the mechanism underlying this tissue difference in the expression level, a comparison is made of their mRNA and heteronuclear RNA (hn-RNA) contents. A 72 bp intron present in the gene encoding SERCA2a/b was cloned and sequenced. By reverse transcription of total RNA followed by PCR using primers based on the sequence of this intron and the cDNA sequences flanking it, a value of 0.37 +/- 0.03 was obtained for the ratio heart hn-RNA/mRNA:stomach hn-RNA/mRNA. Similarly, a ratio of 3.4 +/- 0.5 was obtained for heart:stomach mRNA/28S values. This value was slightly lower but statistically similar to a value of 5.7 +/- 1.8 obtained for the heart:stomach mRNA/poly A+ RNA obtained by Northern blot analysis using a conserved region SERCA2 cDNA probe. Based on mRNA/28S ratio of 3.4 +/- 0.5 and hn/mRNA ratio of 0.37 +/- 0.03, the ratio of heart:stomach for hn-RNA/28S was 1.2 +/- 0.2. Thus, heart which expresses SERCA2a contains 70 +/- 10 times more protein, 3.4 +/- 0.5 times more mRNA and only 1.2 +/- 0.2 times more hn-RNA for this message than the stomach smooth muscle which expresses SERCA2b.