Alpha-1-adrenergic receptor subtypes and signal transduction

• Main Author: Theroux, Tracey L
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1996
• Subjects: Anatomy & physiology; Animals; Cellular biology; Pharmacology; Physiology
• ISBN: 9798209198215

Epinephrine (EPI) and norepinephrine (NE) activate $\alpha\sb1$-adrenergic receptors (AR) causing an increase in cytosolic calcium ( (Ca$\rm\sp{2+}\rbrack\sb{i}$) through at least three $\alpha\sb1$-AR subtypes ($\rm\alpha\sb{1A},\ \alpha\sb{1B},$ and $\rm\alpha\sb{1D}$) and two independent signalling pathways. In general, $\rm\alpha\sb{1A}$-AR couple to Ca$\sp{2+}$ influx through voltage-dependent, dihydropyridine-sensitive (DHP) Ca$\sp{2+}$ channels and $\rm\alpha\sb{1B}$-AR couple to phosphatidylinositol hydrolysis (PI hydrolysis) and release of intracellular Ca$\sp{2+}$ stores. Transfected $\rm\alpha\sb{1d}$-AR also couple to PI hydrolysis, but their physiological role in catecholamine signalling is unclear. There are, however, exceptions to this $\alpha\sb1$-AR subtype/second messenger coupling profile. In rat medullary thyroid carcinoma (rMTC 6-23) cells, which express an apparently homogeneous population of $\rm\alpha\sb{1B}$-AR, we see DHP-sensitive Ca$\sp{2+}$ influx in response to NE. We also found that $\rm\alpha\sb{1A}$-AR couple to PI hydrolysis in both human neuroepithelioma (SK-N-MC) cells natively expressing $\rm\alpha\sb{1A}$-AR (Esbenshade et al., 1993) and in $\rm\alpha\sb{1a}$-transfected human embryonic kidney (HEK) 293 cells. These data suggest that multiple subtypes do not exist because they couple to distinct second messenger systems, as previously thought. To explain the expression of multiple subtypes, we investigated both the selectivity of the natural catecholamines for each subtype as well as relative differences in coupling efficiency to PI hydrolysis. Neither NE nor EPI show selectivity in activating PI hydrolysis through any of the $\alpha\sb1$-AR subtypes. A variety of synthetic agonists (i.e. clonidine and $\alpha$-methyl-NE) and antagonists (i.e. WB 4101 and BMY 7378) are selective for $\alpha\sb1$-AR subtypes, making them useful pharmacological tools, but the lack of selectivity of the natural catecholamines requires further examination of why multiple $\alpha\sb1$-AR subtypes exist. $\alpha\sb1$-AR subtypes demonstrate different coupling efficiencies in activating PI hydrolysis. By modulating receptor expression levels in HEK 293 cells we found that $\rm\alpha\sb{1A}$-AR couple most efficiently to PI hydrolysis, $\rm\alpha\sb{1b}$-AR couple at an intermediate level, and $\rm\alpha\sb{1d}$-AR couple least efficiently. These differences in coupling efficiency may explain how cellular responses to NE and EPI can be modified by changing $\alpha\sb1$-AR subtype expression, and may begin to provide a rationale for the existence of multiple subtypes.