Corticosteroids decrease mRNA levels of SERCA pumps, whereas they increasesarcolipin mRNA in the rat diaphragm

1. In order to explore the potential role of the sarcoplasmic-endoplasmic r eticulum Ca2+-ATPase (SERCA)-type pumps and of their modulators phospholamb an (PLB) and sarcolipin (SLN) in the functional alterations of the diaphrag m induced by corticosteroid treatment, expression of SERCA, PLB and SLN was assessed by RT-PCR in the diaphragm of rats treated daily for 5 days eithe r with triamcinolone (80 mg kg(-1), n = 8) or with saline (control; 0.6 mi, n = 8). 2. Triamcinolone treatment reduced the normalised overall amount of all SER CA mRNA in diaphragm by 70% compared to controls (P < 0.05). This reduction was accounted for by a relatively larger decrease in the SERCA1 mRNA (-69% , P < 0.05) whilst the decrease in SERCA2 mRNA (-49%, P = 0.09) did not rea ch statistical significance. As a result the relative proportion of SERCA2 mRNA was increased from 43 +/- 7% in control diaphragm to 52 +/- 4% after t riamcinolone treatment (P < 0.05). 3. Only the adult isoform of SERCA1 (i.e. SERCA1a) mRNA was found in the di aphragm of the 15-week-old control rats. Furthermore, triamcinolone treatme nt resulted in reduced levels of SERCA2a (-40%, P < 0.05) and increased lev els of SLN mRNA (+100%, P < 0.05), while the decrease in PLB mRNA (-31%, P = 0.277) did not reach statistical significance. SERCA1b, SERCA2b and SERCA 3 mRNA levels fell below the detection limit in the diaphragm of both contr ol and triamcinolone-treated rats. 4. Compared to control diaphragm, control rat heart showed a relatively hig h PLB/(SERCA1 + SERCA2) mRNA ratio of 7.88 while this ratio amounted only t o 0.16 in control extensor digitorum longus (EDL) muscle. Remarkably, the S LN/(SERCA1 + SERCA2) mRNA ratio in normal cardiac muscle (0.96) was nearly the same as in diaphragm, but in EDL it amounted to only 0.05 that in diaph ragm. This indicates the very low expression of SLN in rat EDL. 5. These data reveal that considerable alterations in SERCA mRNA levels acc ompany the functional changes seen in diaphragm after corticosteroid treatm ent. The relatively larger decrease in SERCA1 mRNA is in agreement with the selective type II: fibre atrophy previously observed in the diaphragm of t riamcinolone-treated rats, but the magnitude of SERCA alterations is more p ronounced than expected on the basis of the structural changes in the diaph ragm. The increase in SLN mRNA levels may represent a compensatory mechanis m.