Arterial Muscle Myosin Heavy Chains and Light Chains in Spontaneous Hypertension

• Published in: Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology 1997-05, Vol.117 (1), p.19-28
• Main Author: Packer, C.Subah
• Format: Article
• Language: English
• Subjects: Animals; Arteries; Essential-hypertension; Hypertension - metabolism; Muscle Contraction - physiology; Muscle, Smooth, Vascular - metabolism; Myofibrils - enzymology; Myosin Heavy Chains - metabolism; myosin isoforms; Myosin Light Chains - metabolism; myosin phosphorylation; Myosins - metabolism; Phosphorylation; Rats; Rats, Inbred SHR; Rats, Inbred WKY; vascular smooth muscle contractility
• ISSN: 1096-4959; 1879-1107
• DOI: 10.1016/S0305-0491(96)00311-2

Increased maximum velocity of shortening (Vmax), increased shortening ability (ΔLmax) and decreased relaxation rate have been reported for arterial smooth muscle from 16- to 18-week-old spontaneously hypertensive rats (SHR) compared with age-matched normotensive Wistar-Kyoto rats (WKY). Vmax is dependent on actomyosin ATPase activity, and this activity is in turn dependent on the level of phosphorylation of the 20-kDa myosin light chain (MLC20) normally a function of calcium concentration. In this article, methods are described and data are presented from studies addressing possible intracellular regulatory mechanisms that might lead to the altered contractility of the SHR arterial muscle. In one study, myofibrillar protein was extracted from 16- to 18-week-old SHR and WKY caudal arterial muscle. The Mg2+-activated ATPase activity was measured under conditions where the Ca2+ concentration was controlled. In another study, the amount of myosin present and relative proportions of the myosin heavy chain (MHC) isoforms were determined by quantitative SDS-PAGE using heavy molecular weight standards and bovine serum albumin as the standard for concentration. In a third study, MLC20 phosphorylation levels in electrically stimulated arterial muscle were determined by urea glycerol gel electrophoresis and Western blot analyses. The SHR (n = 6) myofibrillar ATPase liberated 0.011 ± 0.003 μmol Pi/mg myosin/min, which was significantly more than the 0.006 ± 0.001 μmol Pi/mg myosin/min liberated by the WKY (n = 4) myofibrillar ATPase (P < 0.05). Consistent with the increased ATPase activity, phosphorylation of MLC20 was increased by 2.8 times as much in the SHR compared with the WKY electrically stimulated arterial muscle. However, there was no difference in MHC isoform pattern in the SHR compared with the WKY arterial muscle in contrast to the findings of at least one other laboratory. This discrepancy is discussed. The data reviewed in this article lead to the conclusions that an increased actin-activated myosin ATPase activity and MLC20 phosphorylation are likely responsible for the increased velocity of shortening previously reported in SHR arterial muscle and the increased ATPase activity is not a function of an increased myosin content or of altered MHC isoform pattern in the SHR muscle.