Differential regulation on human skin fibroblast by α1 adrenergic receptor subtypes

Alpha 1 adrenoceptor (α1-AR) regulation of DNA synthesis was studied in human neonatal foreskin fibroblast. Saturation assay with a specific radioligand for α1 adrenergic [3H]-prazosin revealed two saturated and specific binding sites with high or low affinity. Competitive binding assay with different antagonist subtypes, defined pharmacologically three major types of α1-AR. The α1-AR agonists (from 1 × 10−10 to 1 × 10−4 M) triggered a biphasic action on DNA synthesis reaching maximal stimulation at 1 × 10−9 M and maximal inhibition at 1 × 10−6 M. Prazosin, abolished the stimulatory (pA2: 9.24) and inhibitory (pA2: 8.80) actions of α1-AR agonists. The α1-AR stimulation resulted in the activation of phosphoinositide turnover (InsP) via phospholipase C (PLC) involving calcium/calmodulin (CaM) and nitric oxide synthase (NOS) that correlates with the DNA synthesis increment; whereas the inhibition resulted in a decrease of cyclic AMP (cAMP) accumulation via adenylate cyclase inhibition. The potency displayed by the specific antagonists tested in binding, DNA synthesis, InsP and NOS at low agonist concentration suggests that they can be elicited by the activation of the same receptor (α1B-AR subtype); while the decrement in DNA synthesis and cAMP at high concentration account by the activation of α1D-AR coupled to Gi protein. Non-functional α1A-AR in neonatal human foreskin fibroblast was observed. Results suggest that the expression of α1-AR subtypes on human skin fibroblast may differentially activate signaling pathways that modulate physiological response of the cells.