Distensibility of left atrium in normotensive and spontaneously hypertensive rats

In spontaneously hypertensive rats (SHR) left atrial mechanoreceptors are reset. Thus, left atrial pressure must be almost twice as high in SHR as in normotensive rats to produce comparable degrees of receptor activation and reflex sympathetic inhibition. The present study was performed to investigate whether this resetting is due to a decreased atrial distensibility in SHR. Static load-length relationships were therefore investigated on isolated left atrial strips from 11 pairs of male SHR and Wistar Kyoto rats (WKR). After each experiment the strips were fixed at a passive tension of 4 mN and the average wall thickness was determined histologically. Furthermore, pressure-volume relationships were studied on non-beating, isolated left atria from SHR and WKR. Distensibility was here defined as % volume increase when LAP was increased from 2.5 to 12.5 mmHg either rapidly (0.5--1 s, "dynamic" distension) or slowly (3 min, "static" distension). Atrial wall thickness did not differ significantly in SHR and WKR, but the passive force (mN) per cross sectional area exerted during elongation above 80% was greater (P less than 0.05) in SHR. Also the "dynamic", but not the "static" volume distensibility was significantly lower in SHR (P less than 0.01). The decreased dynamic distensibility of SHR left atrial walls can at least partly explain the resetting of the atrial receptors activated during the rapid filling phase.