Ion Channels – Membrane Transport – Integrative Physiology
Kidney International (2003) 64, 1348–1355; doi:10.1046/j.1523-1755.2003.00214.x
Increased intracellular Ca++ in the macula densa regulates tubuloglomerular feedback
Yilin Ren, Ruisheng Liu, Oscar A Carretero and Jeffrey L Garvin
Hypertension and Vascular Research Division, Henry Ford Hospital, Detroit, Michigan
Correspondence: YiLin Ren, M.D., Division of Hypertension and Vascular Research, Henry Ford Hospital, 2799 West Grand Boulevard, Detroit, MI 48202. E-mail: yren1@hfhs.org
Received 14 February 2003; Revised 16 May 2003; Accepted 23 May 2003.
Abstract
Increased intracellular Ca++ in the macula densa regulates tubuloglomerular feedback.
Background
Tubuloglomerular feedback is initiated by an increase in NaCl at the macula densa lumen, which in turn increases intracellular Ca++. In the present study, we examined the role of increased intracellular Ca++ in tubuloglomerular feedback and the source of the increased Ca++. We hypothesized that an increase in intracellular Ca++ at the macula densa via the basolateral Na+/Ca++ exchanger, caused by an increase in luminal NaCl, initiates Ca++-mediated Ca++ release from intracellular stores, which is essential for tubuloglomerular feedback.
Methods
Rabbit afferent arterioles and attached macula densas were simultaneously microperfused in vitro. Tubuloglomerular feedback was induced by increasing macula densa Na+/Cl- from 11/10 mmol/L (low) to 81/80 mmol/L (high) and was measured before and after treatment.
Results
To investigate whether elevations in intracellular Ca++ are required for tubuloglomerular feedback, the calcium ionophore A23187 or the Ca++ chelator BAPTA-AM was added to the macula densa lumen. During the control period, tubuloglomerular feedback decreased afferent arteriole diameter from 18.1 
1.1 
m to 15.3 0.8 m. Adding 2 
10-6 mol/L A23187 to the low NaCl macula densa perfusate induced tubuloglomerular feedback; diameter decreased from 18.0 1.0 m to 15.4 0.9 m (N = 6; P < 0.01). After adding BAPTA-AM (25 mol/L) to the macula densa lumen, tubuloglomerular feedback response was completely eliminated. We next studied the source of increased macula densa Ca++ in response to increased NaCl concentration. During the control period, tubuloglomerular feedback decreased afferent arteriole diameter from 18.5 1.6 m to 15.3 1.2 m (N = 6; P < 0.01). After adding the Na+/Ca++ exchanger inhibitor 2'4'-dichlorobenzamil (10 mol/L) or KB-R7943 (30 mol/L) to the bath, the tubuloglomerular feedback response was blocked; however, the afferent arteriole response to angiotensin II or adenosine was not altered. Next, we tested the Ca++-adenosine triphosphatase (ATPase) inhibitor thapsigargin (0.1 mol/L), which has been reported to inhibit sarcoplasmic reticulum Ca++-ATPase activity and prevent restoration of intracellular Ca++ stores. When thapsigargin was added to the macula densa lumen, it reduced the first tubuloglomerular feedback response by 33% and completely eliminated the second and third tubuloglomerular feedback responses. In the absence of thapsigargin, there was no significant decrease in the tubuloglomerular feedback responses (N = 6). Neither the L-type Ca++ channel blocker nifedipine (25 mol/L), nor the T-type Ca++ channel blocker pimozide (10 mol/L), inhibited tubuloglomerular feedback when added to the macula densa lumen.
Conclusion
We concluded that (1) increased intracellular Ca++ at the macula densa is required for the tubuloglomerular feedback response; (2) Na+/Ca++ exchange appears to initiate Ca++-mediated Ca++ release from intracellular stores; and (3) luminal L-type or T-type Ca++ channels are not involved in tubuloglomerular feedback.
Keywords:
Ca++, macula densa, Na+/Ca++ exchanger, thapsigargin, afferent arteriole
