TABLE 5.

Localization of H₂-histamine receptors.

Tissue	Species	Tissue/Cell type: References
1. Cardiomyocytes
1.1	Adult rat	Whole heart: Matsuda et al. (2004) Zeng et al. (2014)
	Neonatal rat	Cardiomyocyte: Zeng et al. (2014)
1.2	Pig	Ventricle: Cooper et al. (1995)
1.3	Human	Atrium and ventricle: Matsuda et al. (2004)
1.4	Mouse	Ventricle: Lacking: Gergs et al. (2019): Present: Fitzsimons et al. (2001)
1.5	Rabbit	Ventricle: Hattori et al. (1991a), (1991b)
1.6	Guinea pig	Ventricle: Agata et al. (2010), Matsuda et al. (2004)
2. Blood cells
2.1	Human	Platelets: Nakamura et al. (1999)
2.2	Human	Mast cells: Bachert (2002)
2.3	Human	Macrophages: Jutel et al. (2009)
2.4	Human	Neutrophils: Busse and Sosman (1976)
2.5	Human	Erythrocytes: Wagner et al. (2006)
3	Human	Vascular smooth muscle cells: Ottoson et al. (1988)
4	Human	Endothelial cells: Luo et al. (2013)
5	Human	Lymphocytes: Jutel et al. (2009)
6	Human	Basophils: Bachert (2002)
7	Rat	Fibroblasts: Zeng et al. (2014)

Here, the tissue distribution and localization of H₂-histamine receptors in different cell types (first column) present in the heart of several species (second column) or blood constituents (third column) are listed. It is apparent that H₂-histamine receptors are by no way confined to mast cells but are present on several cell types. SMC: smooth muscle cells. EC: endothelial cells. It is worth mentioning that whereas the H₂-histamine receptor is found biochemically in the adult rat heart, it is only functional in neonatal and possibly fetal rat heart when one compares this table with Table 4. In the mouse, H₂-histamine receptors were present in wild-type cardiomyocytes as messenger ribonucleic acid by polymerase chain reaction but were functionally absent even in electrically stimulated adult cardiomyocytes (Gergs et al.,2019).