Hypertension Induces Contractile Dysfunction in Rat Mesenteric Lymph Vessels

Abstract

Hypertension predisposes patients to a cluster of deadly cardiovascular and metabolic diseases involving inflammation, dyslipidemia, and fluid imbalance. Current medications fail to normalize blood pressure in 25% of hypertensive patients; therefore, we considered the possibility that the lymphatic circulation provides a new therapeutic target to alleviate hypertension and its associated co‐morbidities. The intrinsic driving force for the propulsion of lymph fluid is the robust, spontaneous rhythmic contractions of lymphatic muscle cells (LMCs) that facilitate the lymph flow which helps maintain fluid balance and the transport of lipids and immune cells. Thus, evaluation of lymphatic contractions and estimation of lymph flow during hypertension is important to understand the possibility of targeting lymphatic circulation in hypertension. Angiotensin II (Ang II) is a key regulator of blood pressure, and increased levels of Ang II are known to induce hypertension. However, the role of Ang II mediated hypertension on lymphatic contractile dysfunction is still a matter of debate. Here, we evaluated the rhythmic contractions of LVs isolated from Ang II infused hypertensive rats (AHT LVs) compared to normotensive controls (NT LVs). Ang II containing osmotic mini pumps (500 ng/kg/min) were implanted subcutaneously in adult (9‐13 weeks old) male Sprague Dawley (SD) rats for 14 days. Rat mesenteric LVs were dissected free from fat and then cannulated and pressurized (4‐5 mmHg) in a perfusion bath filled with physiological salt solution (PSS). Changes in diameter were monitored by real‐time edge detection software. Baseline rhythmic contractions were recorded for ~45 minutes and then a L‐type calcium channel antagonist, nifedipine (NIF; 1 µmol/L) was added to the bath for 10 mins followed by washout with calcium free PSS. AHT LVs had significantly increased calcium dependent tone to 14.3% ± 5.1% (n=8) compared to NT LVs, which remained at 5.2% ± 1.1% (n=8). AHT LVs also exhibited a larger response to nifedipine with 14.0% ± 6.5% (n=6) increase in dilation compared to NT LVs 3.9% ± 0.9% (n=8). Contractile traces also showed visible dysrhythmia, defined as an increased variation in contraction interval, in LVs isolated from AHT rats at 32.4% ± 14.5% (n=8) when compared with NT LVs at 2.3% ± 1.1% (n=8). An increased variance for the time to relaxation also was observed in AHT LVs (7.4% ± 4.6%; n=8) compared to NT LVs (0.6% ± 0.2%; n=8). These differences ultimately resulted in a significant decrease in calculated flow in AHT LVs (62.0 nL/s ± 8.4 nL/s; n=5) compared to NT LVs (92.4 nL/s ± 4.3 nL/s; n=5). Our data suggest that Ang II mediated hypertension can cause lymphatic contractile dysfunction leading to loss of lymph flow and may be a potential mechanism by which hypertension increases the risk of associated co‐morbidities. Studies are ongoing to assess the changes in lymph flow in vivo during Ang II mediated hypertensive versus normotensive rats.