Somatostatin receptor subtype expression in the human heart: differential expression by myocytes and fibroblasts

In acromegaly, somatostatin receptor ligands (SRLs) can ameliorate left ventricular hypertrophy (LVH) and their use is associated with demonstrable improvements in various parameters of cardiac function. It remains unclear as to whether these beneficial effects are principally attributable to falling GH and IGF-I levels, or whether SRLs exert independent direct effects on the heart via somatostatin receptors. To help address this issue, we have sought to investigate somatostatin receptor expression in human heart. A human heart cDNA library was probed using PCR techniques to determine expression of somatostatin receptor subtypes. Subsequently, human heart biopsies and human cardiac fibroblasts and myocytes were analysed to determine whether expression differed between cardiac chambers or cell types. mRNAs for four of the five somatostatin receptor subtypes (sst1, sst2, sst4 and sst5) were shown to be co-expressed by the human heart. These receptors were present in both atrial and ventricular tissue. Human cardiac myocytes expressed mRNA for only sst1 and sst2, while human cardiac fibroblasts expressed all four subtypes found in whole heart tissue. The expression of functional somatostatin receptors on human cardiac fibroblasts was confirmed by mobilisation of intracellular calcium in response to somatostatin. The presence of cardiac somatostatin receptors raises the possibility of a direct effect of somatostatin analogues on the heart. Furthermore, the differential expression of somatostatin receptor subtypes by human cardiac myocytes and fibroblasts opens up the possibility of differential modulation of the cell types in the heart by subtype-specific somatostatin analogues.