The relationship between increased myogenic tone and hyporesponsiveness in vascular smooth muscle of spontaneously hypertensive rats.

Responsiveness of aortic strips from spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) normotensive rats was examined in the presence of different concentrations of Ca++. Helical strips from SHR exhibited a Ca++-dependent myogenic tone, whereas WKY strips maintained a stable resting tension at all levels of Ca++. In the presence of 1.5 mM Ca++, SHR strips were hyporesponsive to norepinephrine (NE) and KCl compared to WKY strips. However, when first exposed to Ca++-free medium, responses of strips from SHR and WKY rats to agonist plus Ca++ were of approximately equal magnitude. When an increase in active tone was induced with KCl in strips from WKY rats, these strips were seen to mimic SHR strips in their responses to Ca++-free solution and to NE. These results indicate that: 1) vascular smooth muscle of SHR possesses intrinsic myogenic tone, possibly due to increased permeability to Ca++, and 2) the decreased responsiveness of SHR strips in the presence of Ca++ is only an apparent hyporesponsiveness which may be attributable to the pre-existing tone.