Cellular Signalling by Lipoproteins in Cultured Smooth Muscle Cells from Spontaneously Hypertensive Rats

Abstract

We investigated pivotal signalling responses of cultured aortic smooth muscle cells (VSMC) from the spontaneously hypertensive rat (SHR) and the normotensive Wistar Kyoto rat (WKY) to lipoproteins. Low-density lipoprotein (LDL) and high-density lipoprotein (HDL₃) stimulated a time- and dose-dependent accumulation of inositol phosphates in VSMC. SHR and WKY VSMC exhibited comparable half-maximal dose requirements (≈13 µg/ml for LDL and ≈14 µg/ml for HDL₃), although, at any given dose, the response of SHR VSMC was significantly greater than WKY VSMC. Simultaneous addition of LDL and HDL₃ to VSMC resulted in additive stimulatory effects on phosphoinositide catabolism. Pertussis toxin pretreatment of VSMC completely negated the stimulatory effects of LDL and HDL₃ on IP accumulation. [³²P]-ADP ribosylation and immunoblotting studies revealed the guanine nucleotide-binding (G protein) substrate(s) for pertussis toxin to be a G_i protein(s). SHR and WKY VSMC did not differ with respect to levels of G_iα or G_β, and, thus, the amplified responsiveness in SHR VSMC cannot be attributed to alterations in levels of pertussis toxin-sensitive G protein. The spectrum of signalling responses elicited by LDL and HDL₃ are similar to those elicited by vasoactive hormones, and thus lipoproteins may, via stimulation of phosphoinositide catabolism, ⁴⁵Ca²⁺ uptake and Na⁺/H⁺-exchange, directly regulate smooth muscle cell growth and contraction.