Left atrial pressure and postprandial diuresis in conscious dogs on a high sodium intake

Uncomplicated human type 2 diabetes is associated with meal-induced blood pressure lowering and cardiac output increase

AIMS

Since many type 2 diabetes patients experience postprandial hypotension, the aim of this study was to unravel meal-related changes in systemic hemodynamics and autonomic nervous system (ANS)-balance.

METHODS

Forty-two age-matched males (15 type 2 diabetes; 12 metabolic syndrome; 15 controls) without overt autonomic neuropathy received a standardized high-fat mixed meal after an overnight fast. Hemodynamic variables were measured by finger plethysmography. Fourier analysis was used to calculate the low-/high-frequency (LF/HF)-ratio, a marker of autonomic nervous system-balance, and baroreceptor reflex sensitivity (BRS).

RESULTS

Following the meal, diastolic blood pressure (DBP) decreased in type 2 diabetes patients only, paralleled by a significant decrement in systemic vascular resistance (SVR) and an increase in cardiac index. All groups showed an increase in postprandial heart rate. Controls, but not metabolic syndrome or type 2 diabetes patients, showed a meal-related increase in LF/HF-ratio. When combining all study subjects, homeostatic model assessment-insulin resistance (HOMA-IR) was inversely correlated with changes in DBP, SVR, LF/HF-ratio and BRS.

CONCLUSIONS

Based on these data, we hypothesize that in patients with uncomplicated type 2 diabetes, insulin resistance hampers adequate meal-induced sympathetic activation, leading to a decrease in SVR and resulting in a postprandial drop in DBP.

No relation between atrial natriuresis and renal blood flow in conscious dogs

Conscious dogs were used to study whether changes in total renal hemodynamics are responsible for diuresis and natriuresis during an experimental increase in left atrial pressure (LAP). To ensure a controlled dietary sodium intake, the dogs (n = 8) were chronically kept on a high or a low sodium intake diet (HSI; LSI). After the dogs had completely recovered from surgery (carotid loop, thoracotomy, flank incision), LAP was increased by about 10 cm H2O for 60 min by tightening a purse string around the mitral annulus (51 expts.). Mean urine volume (V) increased in both groups to a comparable degree. Mean sodium excretion increased somewhat more in HSI dogs, but remained elevated in LSI dogs after the LAP increase. Renal blood flow (electromagnetic flow transducer) and inulin clearance did not change. Renal vascular resistance (RVR) increased by about 20% (HSI) and 15% (LSI). --When the induced LAP INCREASE WAS TERMINATED, V decreased. RVR decreased in HSI dogs by about -11% and in LSI dogs by about -6% below control values. --It is concluded that volume regulatory mechanisms induced by an experimental LAP increase operate independently of changes in total renal blood flow.

Left atrial pressure and sodium balance in conscious dogs on a low sodium intake

10 conscious chronically prepared dogs were used. After recovery from thoracotomy (catheter into the left atrium, nylon purse string around the mitral annulus) they were kept chronically on a low sodium intake (less than 0.5 meq Na/kg bw daily). On 51 days left atrial pressure (LAP) was increased for 60 min about 10 cm H2O once daily by tightening the purse string (distension period: DP). During (DP) urine volume (V) increased about threefold, and sodium excretion (ENa) about sixfold. The amount of renal sodium loss on the days when LAP was increased exceeded the daily intake considerably. The application of DOCA (15 mg i.m.) did not diminish ENa during DP and 60 min therere increased by about 15 mm Hg. The data suggest that stimulation of intrathoracic receptors by a reversible mitral stenosis augments renal sodium excretion even in a state of a highly stimulated tubular sodium resorption.