Quantitative Analysis of Low-Abundance mRNA: Application to Adrenergic Receptors

The adrenergic receptors belong to a family of receptors that is postulated to span the plasma membrane seven times and is linked to regulatory GTP-binding proteins. These receptors mediate a wide variety of physiological responses through activation of several distinct second-messenger systems. Belying the importance of the adrenergic receptors in regulating physiological responses in target cells is their paucity. These receptors are present in very low numbers, in some cases only a thousand copies per cell. In addition, the adrenergic receptors are relatively stable proteins. Thus, cells have no need to synthesize large amounts of these proteins and, in consequence, the level of mRNA coding for these receptors is very low. In this paper, I examine some of the ways in which quantitation of these scarce mRNA species has been approached, with particular emphasis on the use of polymerase chain reaction.